‘Expository Preaching,’ writes Peter Adam, ‘is the preaching of the message of a book of the Bible, usually verse by verse, paragraph by paragraph, chapter by chapter, by explanation and application of it to the congregation.’

Here are fifteen argument in favour of expository preaching:-

1. It explains and applies the Bible as it was actually written – not as a collection of quotations, useful texts, inspiring ideas or isolated stories, but as a sequence of sentences, paragraphs, and book.
2. It reflects God’s respect for human authors: their personalities, circumstances, and styles of communication.
3. It respects the historical context of each part of the Bible.  The truth of the Bible does not float free from its historical context, but is rooted in it.
4. It respects the context of salvation history.  Each passage of scripture has its place within the unfolding drama of redemption, and the preacher should demonstrate where that place is.
5. It helps us to unfold the deep Biblical Theology of the Bible, so that we see every part of Scripture in the light of the gospel.
6. It preserves the shape and balance of the Bible.  Topical preaching, on the other hand, distorts the shape and upsets the balance.
7. It ensures that the preacher does not avoid difficult topics and passages.  Adam says: ‘I would not choose to preach from the text ‘I hate divorce’ unless forced to do so by a sermon series on Malachi. I would not choose to preach on Romans 9-11, but preaching my way right through Romans forces me to do so.’  Unfortunately, says Adam, ‘Lectionaries are no help, because modern lectionaries seem to go out of their way to avoid difficult topics, even cutting poems and stories in half to avoid embarrassment.’
8. It makes efficient use of the preacher’s and hearers’ time.  If preaching through a book of the Bible, the background information needs to be found only once, and then can inform each sermon in the series.
9. It models good exegesis.  We preach the Bible in a way in which we want people to read it.
10. It demonstrates the flow of thought of the human and Divine author of Scripture.  As he works through the Biblical text, the preacher is able to show how the story or argument develops and builds.
11. It is the natural and sensible way of reading a book.  We would not usually dip into a letter, poem, or novel simply in order to get some isolated ‘bon mot’.  So why would we want to make a practice of doing that with the Bible?
12. It teaches people to trust the Bible.  It models an attitude of trust that the Bible is relevant and effective just as it came from the mouth of God.  It does not need our ultra-refinement.
13. It encourages others to teach and preach the Bible.  It encourages suitably-gifted people to take up this method for themselves, and use it in one-to-one work, in small groups, and so on.
14. It dissuades preachers from repeating their own favourite themes.  The expository method helps the preacher to declare ‘the whole counsel of God’ (even though nothing can stop the determined preacher from riding his or her hobby-horse)!)
15. It follows God’s syllabus for us.  The Bible lays out God’s way of salvation, and sets out his requirements regarding faith and obedience.  Why would we replace the divine syllabus with one of our own devising?

Based on this article.