So, in summary, if I was answering a question like “how is Biblical Theology useful for understanding the Old Testament?” I would make the following points (probably with reference to Goldsworthy, Scobie, Dumbrell and Demster):
1. It grounds us in the notion that the Bible is a unified text not a hodgepodge of disparate parts strung together because no other documents existed.
2. It puts other forms of literary criticism in their place as subsets of this view (here I would reference Von Rad, Brueggemann, Barr, the documentary hypothesis and anything else that would demonstrate I knew what was at stake in terms of viewing the Bible as a loose collection of texts).
3. It helps us chart the development of God’s redemptive plans for creation from go to woe. Starting with creation and the fall, through the creation of his people with covenant obligations, the development of kingship, and ultimately culminating with the new creation and Jesus as king.
4. It reminds us that themes carry on, and develop through the Biblical text that are helpful for guiding our exegesis and our preaching.
None of the ideas put forward as unifying concepts for the Bible are entirely satisfying on their own, they all miss something of the complexity of the text (any simplification or summary will inherently do that), but they all play a role in shaping our interpretation. Pointing out linking themes is useful. Provided you don’t want to turn it into the only thread that holds the Bible together.