Here is a great quote from Grant R. Osborne’s book The Hermeneutical Spiral on the importance of historical theology (p. 266):
Historical theology technically belongs between biblical and systematic theology. It studies the way later paradigm communities understood the biblical doctrines and enables us better to understand current theological debates by placing them in bold relief within the history of dogma. The process of revelation is seen in terms of inspiration (the data provided in the Bible) and illumination (the interpretation of that data throughout the history of the church). In this way the theologian gains a critical hermeneutical tool for determining the validity and shape of dogma for the modern age.
At the same time historical theology provides a way out of the tension between biblical and systematic theology, namely a recognition of the proper place of tradition as preunderstanding in the interpretive task. Many have noted the positive value of community understanding (tradition) in providing categories for understanding (so Gadamer). Without traditional dogmas we would fail to catch the implications of biblical passages. Yet at the same time these preformed belief systems can play a negative role when they force biblical statements into preconceived dogmatic categories. The answer is a proper “hermeneutical circle” or spiral within which the text is reconstructed on the basis of our theological system, yet challenges our preunderstanding and leads to a reformation of our tradition-derived categories. The history of tradition greatly aids in this task by placing our theological prejudices in historical perspective and thereby making them more open to influence (and correction if necessary) from the text itself.