Over a hundred years ago, Charles Spurgeron says that the church is becoming increasingly worldly. He stated, "False doctrine and worldliness always go hand in hand, with worldliness leading the way." Without sound theology, people will become unrestraint and self-indulgent hedonist.
As a foundation of truth, systematic theology is necessary if men are to think intelligently and logically about anything. Without this concept, we cannot hold a rational universe. There is no other God, no other truth, nor other possibility, system or meaning outside of Him. God as creator, providential ruler, and redeemer is the necessary cause, connection, will, power, and action between and in all things. Anything short of that is not theology but anthropology. Apart from biblical doctrine one can never properly understand the role of church, family, vocations, or anything else.
Theology helps to unite Christians in truth. True fellowship with God, who is truth, and with God's people requires truth. Furthermore, this truth establishes sound doctrine.
Theology separates truth from error. I believe sound doctrine protect us against false doctrine. Biblically sound theology works to uproot any and all presuppositions and protect against unholy philosophies. In addition to protecting us from errors, some errors are so serious that God calls them destructive. Scriptures tells us that God would curse anyone that would preach anything other than the Gospel.