With that said, I want to offer a few reasons why I preach expositionally and why more preachers should as well.
1. The message of the text is greater than the opinions of the preacher.
Typical among topical pastors is soap box preaching. Every preacher has an opinion and many of them based on the biblical text. Yet often such preaching becomes the ramblings of a man with an office rather than the exposition of the inspired Word of God. Frustrating for me is when a preacher begins his sermon by reading a text and never returning to it. It gives the impression that that text is missing something.
Expositional preaching is one clear way the preacher and the congregation he leads confesses that the gospel found only in the words of Scripture is of greater importance than anything else. We have nothing to say unless it is driven by a sound hermeneutic. Most bad preaching can be traced to a preacher, and a congregation that lets him, who thinks he has something to say. Most good preaching can be traced to a preacher who believes God has already said everything that needs to say.
2. Expository preaching allows the preacher to declare "Thus says the Lord"
Related to the first point, the goal of expository preaching is to do the dirty work of exegesis so that the preacher can boldly declare before his flock, "thus says the Lord." That is the essence of preaching and prophetic ministry. "This is not my word," the preacher says, "but God's."
3. All Scripture is God-breathed, even the difficult texts.
I have grown to love difficult texts. I once had a member of our church come up to me following our service and say, "I can't wait to see what you do with the next passage." I replied, "me either." I knew what was coming next and I knew I had a lot of studying to do the next week. I have found that expository preaching requires the preacher to appreciate and apply the doctrine of the sufficiency and inspiration of Scripture. Some texts are fairly easy to preach. Others are not. Enough experience preaching expositionally and I believe the preacher and the congregation will have a greater appreciation for all of Scripture, not just the popular parts.
4. Expository preaching is Christological.
"The Bible," Martin Luther once said, "is the cradle wherein Christ is laid." His point is that all of Scripture has as its subject Jesus of Nazareth, the Lamb of God. Topical preaching typically begins with the self because it is driven by the preacher. Expository preaching, when done right, begins with Christ because that is where the biblical text directs our attention.
5. Expository preaching takes a load off of the preachers back
I am not sure I could handle not knowing what I am going to preach in advance week-by-week. The stress and difficulty of finding a text or a topic for the next sermon can be extremely difficult and frustrating. When I was an associate minister I would be asked to preach with very little time to prepare. Not knowing what to preach took as much time as preparing to preach. Expository preaching eliminates all of that. Both the preacher and his congregation know what is coming next and can prepare accordingly. This takes a major load off the preachers back and allows him to invest more of his time to the text rather than the search for a text.
6. Expository preaching makes preaching more difficult
Although the previous point remains valid, anyone that has preach expositionally for any period of time will tell you there are weeks where the preacher may want to abandon ship. Every biblical book has its difficult texts and exposition forces the preacher to handle all of them. If you in Exodus, you must preach why Ziporah circumcised her son. If your in Matthew, you have to deal with church discipline.
7. Expository preaching allows the preacher to preach the "whole counsel of God."
As has been suggested above, expository preaching forces the preacher to preach all of Scripture, not just the parts he prefers. In Acts 20:27, Paul told the Ephesians that he did not shrink from proclaiming to them the whole counsel of God. That should be the goal of every preacher. Expository preaching is the best method for doing so.
8. Expository preaching tells the whole story of Scripture.
The Bible is not a collection of stories, but is a story. Topical preaching often fails in showing this. When the preacher goes verse-by-verse he must handle the texts immediate context and biblical context. One cannot properly exegete the New Testament without understanding and proclaiming the story of the Old Testament.
9. Expository preaching teaches the congregation how to read and interpret the Bible.
No doubt our congregations are malnourished. Most do not read their Bibles nor know how to. Poor preaching and pastoral leadership is to blame here for much of it. When preachers read just one verse and never exegete it our congregation falls in line. Many might tweet verses that sound inspirational but are utterly ignorant of its context or what it actually means. Expository preaching is one means of remedying this.