What is so special about preaching? Why has it been assumed for so long that it’s a crucial part of the corporate worship experience of the church? I imagine many of us don’t think about this very much because it’s such a given in the tradition- preaching is just what happens after the right amount of singing (whatever that is!) and after the offering is collected. When important things are left unspoken for long they become tacit assumptions. This isn’t always necessarily wicked or stupid, but to leave the reasoning for different aspects of church practice unexamined will almost always result in a fossilized formalism that leaves power & substance by the wayside. This type of practice can’t appeal to any basis other than “it’s always been done this way.” It kicks up a bunch of dust but doesn’t have much to show for itself- it maintains an appearance of godliness but that’s really about it (2 Timothy 3:5). So, let’s inquire: what is preaching, and what is it for? At the end of the day, what makes it a necessary & profitable part of what being the church means?
First off, what is it, at bottom? Sidney Greidanus has a pretty amazing answer:
Preachers today are neither Old Testament prophets nor New Testament apostles. Unless one would be guilty of both presumption and anachronism, one must constantly keep in mind the great difference between preachers then and preachers now. Preachers today do not receive their messages directly from God the way the prophets did. Nor can preachers today claim with the apostles that they were “eyewitnesses” (2 Pet 1:16; cf. Luke 1:2). And yet, provided their sermons are biblical, preachers today may also claim to bring the word of God.
As we noticed in the partial shift from the direct revelation of vision or audition received by the prophets to the exposition of Scripture by the apostles, one does not necessarily need direct revelation in order to speak God’s word- God can speak his word mediately, by means of the exposition of prior revelation. One need not even be an eyewitness of Jesus’ life, death, and resurrection in order to speak God’s word- Paul can instruct young Timothy to “preach the word” (2 Tim 4:2; cf. 1 Cor 16:10). In order to demonstrate that preachers other than apostles can bring God’s own word, Klaas Runia appeals particularly to 2 Cor 5:18-20, where Paul writes that “God gave us the ministry of reconciliation.” Along with many commentators, Runia suggests that with the word us Paul means himself and “‘his assistants’ or ‘other preachers of the Gospel.'” If this identification of “us” is correct, argues Runia, “it also means that the following words apply to all preachers of the Gospel: ‘We are ambassadors for Christ, God making his appeal through us.’ If today’s preacher brings the same message of reconciliation as Paul and the other apostles, God also speaks through him. Then his word too is not just a human word, but the Word of God himself.”
Even more than the apostles, of course, today’s preachers are dependent on the Scriptures as their source of revelation. The sermon as an exposition of the Scriptures can trace its roots from the Old Testament priesthood (Deut 31:9-13; Neh 8:1-8) to the synagogue (see Luke 4:16-27; Acts 13:14-41; 17:1-3) to the New Testament church. Some have sought to articulate the difference between the biblical preachers and their contemporary counterparts as follows: “The Old Testament and the New Testament organs of revelation came forward, saying: ‘Thus says the Lord.’ …But the New Testament preacher must say, if he would speak strictly: ‘Thus has the Lord written.'” Technically, in terms of the source of revelation, this formulation is correct, but materially, in terms of the reality of God’s word, contemporary preachers should also be able to say: “Thus says the Lord.” For the Spirit who spoke through the prophets is still speaking today through preaching which passes on the messages of God’s prophets and apostles. Although the Spirit’s speaking is by no means limited to preachers (think of parents, teachers, friends, and neighbors through whom the Spirit speaks today), contemporary preachers have a special responsibility to proclaim the word of the Lord. No less than their biblical counterparts, contemporary preachers are called to be channels of the word of God. The metaphors of herald and ambassador apply as much to them as they did to the apostles. This high view of preaching came to clear expression in the Reformed Second Helvetic Confession of 1566: Praedicatio verbi Dei est verbum Dei (the preaching of the word of God is the word of God).
But if God speaks through contemporary preachers, then this word of God is also God’s deed today, a redemptive event. This view reflects Paul’s amazing statement that the gospel is “the power of God for salvation to every one who has faith” (Rom 1:16; cf. 1 Cor 1:18). Contemporary preaching of the gospel, therefore, is an indispensable link in the chain of God’s redemptive activity which runs from Old Testament times to the last day (Matt 24:14). God uses contemporary preaching to bring his salvation to people today, to build his church, to bring in his kingdom. In short, contemporary biblical preaching is nothing less than a redemptive event.
This high view of preaching can never be the boast of preachers, of course; it can only underscore their responsibility. For with the prophets we noticed that their authority did not reside, ultimately, in their calling or office but in the words they spoke, whether they were from the Lord. So it is with preachers today: they have a word from the Lord, but only if they speak the Lord’s word. The only norm we have today for judging whether preachers speak the word of the Lord is the Bible. 
Karl Barth upholds this high view of preaching offered in the Second Helvetic Confession and offers a dialectical, incarnational definition based upon it:
1. Preaching is the Word of God which he himself speaks, claiming for the purpose the exposition of a biblical text in free human words that are relevant to contemporaries by those who are called to do this in the church that is obedient to its commission.
2. Preaching is the attempt enjoined upon the church to serve God’s own Word, through one who is called thereto, by expounding a biblical text in human words and making it relevant to contemporaries in intimation of what they have to hear from God himself.
Externally these two formulas contain all that we have come up with thus far. The same elements occur in both, each in its own place, each seen from the decisive standpoint. Together they form the answer to the question of the relation between the Word of God and the human word.
The totality forms a closed circle which begins with God and ends with him. 
His definition recognizes that exposition is a crucial component of what preaching is but doesn’t reductively identify it with exposition, i.e. it’s important, but it just isn’t the point of preaching. The confusion of the two seemed to get him a little miffed:
It has now become a most unusual consideration, common only in the language of edification, to say that people go to church to hear God’s Word- no, they go to hear Pastor So-and-So– or to say of the pastor that his task is to proclaim God’s Word- no, it is to offer his expositions, meditations, applications, and demands! I need hardly say that the devastating lack of tension and dynamic, the lukewarm tediousness and irrelevance of Protestant worship, is closely connected with the disappearance of this consideration. The lack might be concealed or reduced by good preachers, but let us not be deceived: the ship is leaking even though the best preachers be at the pumps. There is no lack of good preachers and sermons, but a lack of sermons that are meant to be God’s Word and are received as such- a lack of qualified preaching. 
Barth’s view makes preaching something of a sacramental act, i.e. an earthly means by which God’s grace effectually penetrates into our lives . Barth views preaching as a redemptive event because in it God reshapes reality through Christ-centered, new creation speech acts , as Frank Rees unpacks in the following:
A sermon is an activity of God, at least that is what it hopes to be. Through the preaching and the listening, God speaks. Barth in fact goes further, to say “God is present”. The sermon may be a sacrament of God’s real presence just as much as is the Lord’s Supper. The sacrament of preaching is an activity of God, and one of the other things we can say about God is that when and as God acts, our reality changes, or at least our perception of our reality changes…
The power to name things is in fact not just a linguistic power, it’s also the power to set things in place, to define where they belong, even to define who they are. This power of naming is given to human beings in the Genesis 1 creation story, for example. We name our world and when we name it, it is our world. But a sermon can change this: “Preaching can re-name the world ‘God’s world’ with metaphorical power, and can change identity by incorporating all our stories into ‘God’s story’.”
In this sense, then, a sermon is an activity of God, not just about God, and not just words. It is an activity in which God changes our world. It is an act of re-interpretation and re-constitution. Reality, the reality we experience, changes as our relationship to things and to God is transformed. The whole shape of things is changed by the sermon as sacrament. 
How often do we view the preaching in our churches in these terms? Good night! I would probably never have taken a crack at it if I had known it to be this weighty a thing! Maybe preachers ought to tremble a bit more before climbing up to the pulpit ready to wave handkerchiefs & kick the podium but they would first have to recognize that preaching is a prophetic act and not their weekly soapbox rant session. No, it is so much more! Preaching is God’s address to His people, corporately gathered.
Moving on, then, to our second question: what’s it all about? What is it for? What end should preachers have in view as they shape their sermons? We’re on our way to that answer with Greidanus’ & Barth’s definitions of preaching, but Bryan Chapell makes it clear as day that above all else it is to minister the grace of Christ to its listeners:
Since God designed the Bible to complete us, its contents necessarily indicate that in some sense we are incomplete. Our lack of wholeness is a consequence of the fallen condition in which we live. Aspects of this fallenness that are reflected in our own sinfulness and in our world’s brokenness prompt Scripture’s instruction and construction. Paul writes, “Everything that was written in the past was written to teach us, so that through endurance and the encouragement of the Scriptures, we might have hope” (Romans 15:4). The corrupted state of our world and our being cry for God’s aid. He responds with his Word, focusing on some facet of our need in every portion. Our hope resides in the assurance that all Scripture has a Fallen Condition Focus (FCF). God refuses to leave his frail and sinful children without guide or defense in a world antagonistic to their spiritual wellbeing. No text was written merely for those long ago; God intends for each Scripture to give us the “endurance and the encouragement” that we need today. The FCF is the mutual human condition that contemporary believers share with those to or for whom the text was written that requires the grace of the passage. 
Chapell clarifies that sin isn’t the only FCF of a passage: “An FCF need not be something for which we are culpable. It simply needs to be an aspect of the human condition that requires the instruction, admonition, and/or comfort of Scripture” . While it would be true to say that God sends His Word to critique and to comfort, because God is revealed most definitively in the person of the Redeemer, Jesus Christ, it would be more accurate to say that He critiques in order to properly comfort. This reinforces Greidanus’ point that preaching is a redemptive act: in it Christ is offered as the solution to our needs. This being the case, it ought to be characterized above all else by grace: “The text’s FCF defines God’s mercy as it reveals human need… God’s imprinting of our incompleteness on a passage of Scripture does not merely demonstrate an aspect of our fallenness; it also reveals the nature and character of him who must make us whole” . The preacher’s task is to unburden his listeners by using the microphone of Scripture to verbally portray Christ; as we see him more clearly we are transformed more & more into his likeness (2 Corinthians 3:18) . The preacher doesn’t have to jump over hedge & ditch, Spurgeon-style, to arrive at Christ, though- he only needs to examine Scripture’s metanarrative for types, longitudinal themes & fulfillment patterns to see how individual texts point to the redemption & restoration accomplished by Jesus:
A passage retains its Christocentric focus, and a sermon becomes Christ-centered, not because the preacher finds a slick way of wedging a reference to Jesus’ person or work into the message but because the sermon identifies a function this particular text legitimately serves in the great drama of the Son’s crusade against the serpent. 
The modern preacher patterns his preaching after the apostles’, whose witness sprang out of Christological meditations on biblical texts. They anchored their preaching and teaching in the Christ event, clearer insight into which is gathered through what God has said/done  in prior revelation, then brought it to bear upon the concrete, up-to-our-eyeballs-in-hardship life situations of their listeners. Preachers here and now must do the same: offer Jesus by contextualizing God’s authoritative speech-acts in to fit the less-than-ideal circumstances their people are embroiled in.
Bringing these answers together brings us to a more God-honoring, grace-expectant, and dynamic understanding of what preaching (and the entire corporate worship service) ought to be: the congregation gathers to hear God address them with His word of reconciliation (past, present, and future), an address which applies Jesus’ accomplished redemptive work to His floundering, exhausted people. In faithful preaching we hear God reaffirm His testimony , hear Him reassure us of His love and acceptance and protection. Isn’t this only fitting, when we remember that Jesus is Immanuel, God is with us? It seems like what we usually lose sight of is that because of Jesus God is for us. This pledge is the most central core of what the gospel is, and it- not religious devotion or good works or extensive knowledge of systematic theology or a militant us vs. them culture warrior outlook- is the only fuel and engine for the Christian life.
 Sidney Greidanus, The Modern Preacher and the Ancient Text: Interpreting and Preaching Biblical Literature (Grand Rapids: Eerdmans, 1988), pp. 7-9
 Karl Barth, Homiletics (Louisville: Westminster/John Knox Press, 1991), p. 44
 Karl Barth, The Gottingen Dogmatics: Instruction In the Christian Religion (Grand Rapids: Eerdmans, 1991), I:31
 “The best preaching is as such an equivalent to the kerygma that the Roman Catholic church offers every day in the form of the sacrament of the altar. According to the conscious and universal view of this church, that sacrament is the living Word of God today, the daily renewal, presentation, and offering of what took place at Golgotha… If the presupposition is lacking [i.e. “the personal, oral, responsible witness to the truth” of God’s speaking in Scripture], the advantage of Roman Catholicism is unmistakeable. The sacrament as the visible Word of God is simply more and better than the finest or most eloquent Protestant address as such if it does not have this character” (ibid., p. 31).
 Saying and doing are the same thing for God who acts via His word. The biblical identification of God’s words with His acts anticipates the modern concept of the speech-act, for which see Anthony Thiselton’s The Two Horizons & New Horizons In Hermenteutics: The Theory and Practice of Transforming Biblical Reading and Kevin Vanhoozer’s Is There a Meaning In This Text? The Bible, the Reader, and the Morality of Literary Knowledge.
 Frank D. Rees, “The Need and Promise of Christian Preaching,” The Evangelical Quarterly 66:2 (1994), pp. 118-119
 Bryan Chapell, Christ-Centered Preaching: Redeeming the Expository Sermon [Grand Rapids: Baker Books, 1994], pp. 41-42
 ibid., p. 43
 ibid., p. 271
 The alternative: “Simply railing at error and hammering at piety may convince others of their inadequacy or callous them into self-sufficiency, but these messages also keep true godliness remote” (p. 285).
 Chapell, Christ-Centered Preaching, p. 293
 There’s speech-act theory again!
 “And this is the testimony, that God gave us eternal life, and this life is in his Son” (1 John 5:11).