The concept of biblical theology used to puzzle me. To some extent, it still does because there is a real sense in which, as D.A. Carson says, "Everyone does what is right in his or her own eyes and calls it biblical theology". This makes understanding biblical theology and its distinction from systematic theology difficult to discern. Yet they are indeed distinct from each other, though not at odds with each other and it seems like the best place to see that in practice is at the pulpit.
Many pastors who are committed to an expository philosophy of preaching will choose to structure a series of Sunday sermons by going through a book of the Bible. That is not to say that expository preaching necessitates this (one can guest preach a stand-alone message and it still be expository in nature) though that is indeed a common practice. This is the practice of my pastor (currently journeying through Luke) as well as the pastor of the church my wife and I attended back in Charlotte, NC (currently journeying through Hebrews). They will preach on a section of a given book and then the next week they will inch on to the next and so on. The text in view for that week is the central driving text for the message. It is the primary text being expounded upon and the focus of the text is sought to be the focus of the sermon. And as the weeks go by, this series of primary texts begin to paint a picture of the unfolding narrative/argument/image that is being revealed in the given book.
For instance, a pastor preaching through the Gospel of John will likely point back to Peter's three denials of Jesus in chapter 18 when expounding on Jesus' reinstatement of Peter in chapter 21. This is because the pastor understands that to properly understand chapter 21 you need to see it in its narratival relation to chapter 18 and vice versa. This is a biblical-theological instinct. Yet at the same time, this same pastor will likely consider what the whole Bible says about what it means to love God and likely will comment on the various Greek terms for love throughout the New Testament. This is a less narratively/redemptive-historically influenced theological question and a more timeless one which thus lands it in the realm of systematic theology. This is using D.A. Carson's helpful explanation of the distinction between biblical theology and systematic theology which I have strategically withheld until now,
Systematic theology asks and answers atemporal questions such as "what is God like" and "what did the cross achieve". All of those questions are asked in atemporal categories. Whereas biblical theology always asks and answers its questions along the axis of the Bible's storyline; along the axis of redemptive history. It doesn't ask what is God like, it asks "What did the prophecy of Isaiah contribute to the unfolding doctrine of God?"
The presence of both of these areas of theological thinking makes good sense to us on a Sunday morning. If our pastor is journeying through a book of the Bible, we would want his later sermons to look back to the earlier ones (and vice versa to an extent). That is because the books of the Bible (and the Bible as a whole) are internally harmonious and harmonious with each other and so you can trace a narrative/argument unfolding within them and with respect to their position with the redemptive-historical framework. Yet, at the same time, Scripture gives us theological truths that can be appraised and systematized into doctrinal categories such as hamartiology (the theology of sin) and ecclesiology (the theology of the church). Both are good and necessary yet simply ask different questions.