There are three reasons to study theology, for having a "head knowledge" of the gospel. I would not say that any are more important than the others, but here they are.
The first two are related - to defend the faith against attacks, and to bring others to Christ. This could be summed up as instructing, whether actively to convert, or passively to defend. The third reason is to gain a richer understanding of your own faith. Like so many things in Christianity, these different reasons can work together in amazing ways. First, let's use it to look at sin.
It's important to understand exactly what sin is. Sin is, at its core, rebellion against God. It's not just about doing something which is "just wrong", but in showing that this thing matters more than God's commands. All sin is, in its own way, idolatry, because it places something else above God - usually some form of pleasure. This is why sin is bad: because nothing is above God. He is supreme, and to treat something else as more important than him is, simply put, blasphemy. So, the actions of sin are idolatry, which comes from a rebellion against God's supremacy.
How much more great does God's grace become? He is not just letting you off for violating some arbitrary rule designed to restrict you, but He is forgiving you for claiming that something which He created is more important than Him, the One who created both that thing, and you.
Knowing this then has two benefits: not only can you use it to explain the severity of sin, and why we must follow those rules, but you can use it to greater appreciate the grace of God in forgiving sin, and even better know why you must follow the law which He has laid out.
Another example, perhaps even more important, is love. Love, of course, is a word used to mean many things, but here I would say its true meaning is to seek what is best for that which you love. Consider that beautiful verse, John 15:13. "Greater love has no one than this: to lay down one's life for one's friends." The purest expression of love is sacrifice, to seek what is best for your friends even at the cost of your own life. Love is more than just an emotion, it is an act, a commitment. When you say you love someone, you say "I want what is best for you".
Now, here's the clincher, and this ties back to my point about sin: God loved you, He wanted what was best for you, even when you actively rebelled against Him. When you committed the atrocity - and I say "you", because everyone has done it - when you committed the offence of claiming that something else was greater than God, when you pushed Him away and tried to reduce Him to something less than His own creations, He still loved you! He still wanted what was best for you. It's like being offered a choice between the basest of trash and someone's most prized possession, choosing the trash, and spitting in the face of the one who gave it to you, but multiplied infinitely, and yet God still loves you. He still wants to give you that most prized possession. Not only that, but when you understand the extent of His love - that He loved you so much He sent His own Son to die for you, despite the nature of your crimes, even though every sin you commit is spitting in His face, how can you help but praise Him?
This is theology. Knowing not just the existence of sin and love, but what they are, and the implications of this. The more you learn about God, about His grace and His love and even His justice, the more you can appreciate Him, and the better you can spread His word. It gives you both a deeper understanding, and a more solid foundation for your faith.
To dismiss "head knowledge", saying that all you really need is the love of God, is to miss out on so much of that love. Learning these things and thinking on them is itself a labour of love. It will bring you closer to God, and perhaps help to bring others closer to Him. Whether you study the Trinity, the role of elders in the Church, the real-world benefits to the 10 commandments, you will better appreciate God, and more importantly, be better able to glorify Him.