Theology equips us to refute error. You can spot the bad by studying the good (Titus 1:9). If you want to identify a false gospel, study the biblical gospel. In the Secret Service, they invest time studying authentic currency in order to identify the counterfeit.
Theology gives help on understanding modern issues. A theology of manhood and womanhood will inform you on the debate over transgenderism and homosexual marriage. A theology of creation will inform you on the place of science in our Christian faith. A theology of authority and governments will inform you on the role of Presidents, the military, local law enforcement, etc. To defend the faith (Jude 3) is to know the faith given to us in His Word.
Theology is part of the Great Commission. Jesus commands us to teach everything He has taught (Matthew 28:16-20). Jesus was a theologian who taught from the O.T.
Theology leads to spiritual growth. Orthodoxy leads to orthopraxy. Doctrine leads to duty. Application is preceded by right interpretation. True spiritual growth is built upon the right belief system.
Theology synthesizes Scripture. When you are asked, “What does the Bible teach about angels?”, the only way to give a confident answer is to look at all passages that reference angels. That is what we might call systematic theology. This enables us to know what the Scripture has to say on any given topic.
Theology tells us what we need to know about God. God wants us to know Him and He has told us about Himself (Hebrews 1:1-2; II Peter 1:20-21). But to know Him, we must read of Him. And to read of Him is to read theologically.
Theology unifies the church. When we can agree on doctrine, we share an intimacy that is un-worldly (Ephesians 4:1-16).