Cornelius Van Til on why individual Christians, whatever role they play in the body, ought to give themselves to systematic theology:
The unity and organic character of our personality demands that we have unified knowledge as the basis of our action. If we do not pay attention to the whole of biblical truth as a system, we become doctrinally one-sided, and doctrinal one-sidedness is bound to issue in spiritual one-sidedness. As human beings we are naturally inclined to be one-sided. One tends to be intellectualistic, another tends to be emotional, and still another tends to be activistic. One tends to be only prophetic, another only priest, and a third only king. We should be all these at once and in harmony. A study of systematic theology will help us to keep and develop our spiritual balance. It enables us to avoid paying attention only to that which, by virtue of our temperament, appeals to us. (An Introduction to Systematic Theology, p 22)
Not only does systematic theology aim at harmony within the individual believer, but according to Van Til, “a thorough knowledge of the system of truth in Scripture is the best defense against heresy” and “also the best help for the propagation of the truth.” In other words, it’s the best foundation from which to love your neighbor–those who love the truth will be encouraged, and those who reject the truth will actually be rejecting an accurate, coherent presentation of the truth.
Van Til concludes:
[W]e should observe that just as a thorough knowledge of the system of truth in Scripture is the best defense against heresy, so it is also the best help for the propagation of the truth. This is but the other side of the former point. As an army well organized is not so likely to be overcome by a surprise attack and is not so likely to be shattered as an army poorly organized, so also an army well organized is better able to attack the enemy than an army poorly organized. Each unit will have the support and the protection of the whole army as it goes on to the attack. The morale will be better. When the enemy comes with cannon, we must be able to put atomic bombs over against them. When the enemy attacks the foundations, we must be able to protect these foundations.
The church will have to return to its erstwhile emphasis upon its teaching function if it is to fulfill its God-given task of bringing the gospel to all men. Its present recourse to jerky evangelism as almost the only method of propaganda is itself an admission of paupery. It is remarkable that what the church, generally speaking, still does in the way of teaching is shot through with modernism. (An Introduction to Systematic Theology, p 24)
To which, I would only add that the best place to start learning systematic theology is in the historic creeds and confessions of the church. They’re not perfect, but they are all kinds of helpful, especially when you read through all the cited scriptures as you go. The best confession I’m aware of is the Second London Baptist Confession of Faith from 1689, which can be found here: http://www.vor.org/truth/1689/1689bc00.html