I'm still asked upon occasion why my sermons are almost exclusively expository. The expository sermon is an exposition of a biblical text in context. The sermon text for an expository sermon is commonly about the length of a paragraph, but it might also be an exposition of one or two verses, a single phrase, a whole chapter, or even an entire book of the Bible. There are several reasons to choose expositional preaching over topical preaching.
1) Expository preaching keeps me tied to the text. With topical preaching one might easily begin with a premise one assumes to be true. The Bible is then ransacked for verses to back up the premise that was already predetermined. The big risk is looking to Scripture to back up what you believe as opposed to letting Scripture continue forming and refining your beliefs. Topical preaching puts the preacher in control of the text. Expository preaching makes it more likely that the text will control the preacher and keep him in check.
2) Expository preaching forces me to deal with issues I might otherwise neglect. Suppose I am preaching through the entire Book of Romans. That forces me to deal with chapters 9-11. If left to my own discretion I would probably choose chapters 1, 3, 6, 8, and 12. But chapters 9-11 would never be heard from the pulpit. The whole of Scripture needs to be preached. Expository preaching can discipline one to make sure the more difficult and controversial texts receive a fair hearing.
3) Expository preaching keeps authorial intent before me. It is more likely that a biblical author's intent will be heard when passages are preached in context with attention to historical background and literary structure. But topical preaching tends to weave numerous verses together into an artificial tapestry of the preacher's own construction. The premise could be an agenda that never crossed any biblical author's radar screen. The Bible is reconstructed to support the preacher's own agenda. When teaching Scripture, more is not always better. What makes a sermon biblical is not the number of Scriptures quoted, but whether Scripture is shown to be understandable and relevant. I've heard too much topical teaching in which one biblical author is invoked to effectively cancel out another. Expository teaching increases the likelihood that we'll follow an individual author's development of thought before comparing him with other authors who wrote to address different situations in different contexts.
4) Expository preaching is practical. While listening to an expository sermon, people don't have to keep flipping around in their Bibles to keep up with the preacher. Everyone camps together in the same passage to see how what the passage said in its original context still has a claim on our lives today. Even one unfamiliar with Scripture can go home, re-read the text, and be able to summarize what it means and how to apply it.