In the article "Preaching and Biblical Theology" in The Southern Baptist Journal of Theology (the theological journal of Southern Seminary), Schreiner provides a helpful perspective for why biblical theology is so necessary and foundational for preaching the Bible:
If we do not preach the OT in terms of the whole canon, we will either restrict ourselves to moral lessons from the OT, or, what is just as likely, is that we will rarely preach from the OT. . . . if we do not preach the OT canonically, in light of biblical theology, it will too often be passed over in Christian preaching. And in doing so, we not only rob ourselves of wonderful treasures from the word of God, and but we also fail to see the depth and multifaceted character of biblical revelation. We put ourselves in a position where we do not read the OT as Jesus and the apostles did, and hence we do not see that the God’s promises are yes and amen in Jesus Christ.You can read the entire article here.
Reading the OT canonically does not mean that the OT is not read in its historical cultural context. The first task of every interpreter is to read the OT in its own right, discerning the meaning of the biblical author when it was written. Further, as we argued above, each OT book must be read in light of its antecedent theology, so that the storyline of scripture is grasped. But we also must read all of scripture canonically, so that the OT is read in light of the whole story—the fulfillment that has come in Jesus Christ. We always consider the perspective of the whole, of the divine author in doing biblical theology and in the preaching of God’s word. We read the scriptures both from front to back and back to front. We always consider the developing story as well as the end of the story.
Our task as preachers is to proclaim the whole counsel of God. We will not fulfill our calling if as preachers we fail to do biblical theology. We may get many compliments from our people for our moral lessons and our illustrations, but we are not faithfully serving our congregations if they do not understand how the whole of scripture points to Christ, and if they do not gain a better understanding from us of the storyline of the Bible. (1o/2: Summer 2006, 97-98).