A pastor’s regular preaching should be expositional because God’s Word is what gives life to dead sinners and sanctifies struggling saints (1 Pet. 1:23; Jn. 17:17). God’s Word is what convicts, converts, sanctifies, and builds up his church. A regular diet of anything other than expositional preaching actually hinders the work of God’s Word because it fails to tell the church what God has said, all that he has said, and only what he has said.

1. A pastor’s regular, week by week preaching should be primarily expositional. Only by explaining and applying the meaning of successive biblical texts can a preacher let God’s agenda lead the congregation, rather than his own.
2. A pastor’s regular preaching should be expositional because God’s Word is what gives life to dead sinners and sanctifies struggling saints (1 Pet. 1:23; Jn. 17:17). God’s Word is what convicts, converts, sanctifies, and builds up his church. A regular diet of anything other than expositional preaching actually hinders the work of God’s Word because it fails to tell the church what God has said, all that he has said, and only what he has said.
3. A pastor’s regular preaching should be expositional because only by preaching sermons in which the point of a biblical text is explained and applied can a pastor teach his flock how to read the Bible and profit from it themselves.
4. A pastor’s regular preaching should be expositional because a preacher will grow most in his knowledge of God and his Word by diligently studying God’s Word each week in order to understand it and apply it to his congregation. Other kinds of preaching essentially allow a preacher to preach what he already knows and pick Bible verses to back it up. Expositional preaching requires the preacher to wrestle with a different text every single week, which will sharpen and challenge him in ways that would never happen otherwise.

Source