Before we can understand why systematic theology is essential, we must first understand what it is. There's no single definition of systematic theology, but at its heart it's the discipline captured by the phrase "faith seeking understanding."
Systematic theology builds on the results of biblical theology. Biblical theology is the exegetical discipline that seeks to grasp the entirety of Scripture as the unfolding of God's plan from Genesis to Revelation. Starting with Scripture as God's Word written through human authors—our final authority (sola scriptura) for what we think about God, ourselves, and the world—biblical theology seeks to "put together" the entire canon in a way that's true to God's intent.
Systematic theology then applies the truths gained in biblical theology to every aspect of our lives. It leads to doctrinal formulation—what we ought to believe and how we ought to live—warranted by the canon and done in light of historical theology.