I could give a list of reasons that I give in talks about this matter, but simply, I believe that expository preaching is so significant because 2 Timothy 4:1–2 comes after 2 Timothy 3:16–17. The end of 2 Timothy 3 says, “All Scripture is breathed out by God, and is profitable for teaching, rebuke, correction, and instruction in righteousness.” On the other side of that, built on the foundation of the nature, the truth, the authority, the clarity, and the sufficiency of God’s Word, Paul issues the charge to Timothy to preach the Word (2 Timothy 4:1–2).
How a man preaches betrays his conviction about the Bible. If the Bible is the Word of God, there is no other faithful way to preach it than to seek to understand what the Scripture means and faithfully explain it to the saints of God, exhorting them to live in faith and obedience.
Expositional preaching is not just another style of preaching that we can choose from, where one chooses this way and one chooses another way. Exposition matters because preaching reflects convictions about Scripture, and exposition is the most faithful way to let the Word of God speak for itself.