I like Walter Liefeld’s explanation of why expository preaching is so important:-

1. it conveys the biblical revelation of God and his will, protecting the sermon (and its hearers) from the preacher’s subjectivism.
2. it teaches God’s word in the setting chosen by the Holy Spirit.  It anchors the preacher’s message in its contextual intention.
3. it meets human needs.  It allows God’s word (rather than merely my own thoughts) to reach hearts.
4. it directs the attention of the hearer to the Bible.  It teaches people to go to the Bible as their own resource and authority.  Expository preaching models this.
5. it feeds the congregation in a way that trite or scholastic preaching does not.
6. it serves as an important protection against improper interpretation of Scripture, including spiritualization.

Liefeld, New Testament Exposition, 7-10.

Similarly, Peter Adam urges:-

1. ‘Expository sermons help us to let God set the agenda for our lives.’
2. ‘Expository preaching treats the Bible as God treated it, respecting the particular contexts, history and style of the human authors.’
3. Expository preaching ‘gives ample time for us to make clear the context of the Bible passage from which we are preaching.’

Speaking God’s Words, p128.