Here's my response.
I think to understand the reasoning behind any sort of art theory you need to examine the definition of art (my definition, anyway) which is any form of communicating ideas or emotions that appeals to the senses or to the soul. I'd compare not knowing theory to not knowing a language, and being fluent in theory to being fluent in language. When you were too young to talk, you were pretty successful at communicating basic emotions and needs without knowing any sort of language. You just screamed, or cried, or laughed, or made a funny face, and you communicated. But when you learned a language, the precision and detail with which you could communicate what you wanted increased greatly. Now turn to music. You can make music without music theory, you can communicate quite effectively without music theory. If you're extremely talented, you can communicate pretty much anything without music theory. However, what music theory does is makes it easier to communicate.
You probably hear a lot of music in your head. Without music theory, you would either be able to play it with some varying degree of accuracy (depending on how talented you are), or it would be lost forever when you forgot it. But if you know theory, you could take the music in your head and write it down, and quickly insure that it's preserved. I have a binder full of ideas that I discovered and wrote down.
Theory helps you recognize areas of dissonance where you might have missed it otherwise, it helps bring order to compositions, helps keep things organized, and most importantly, it helps you give your ideas to other musicians so they can play them as well. Writing a symphony would be a near-impossible task if you didn't know theory. Not only could you not keep the parts organized on a page, but you would have to make sure that each musician plays the right thing at the exact right time every time, and you would have to make sure all the parts sound how you want them to sound, every time.
I got into composition before I got into music. Before I knew any theory, I would plunk out little melodies on the piano. I had a bit of a talent for this, which is why I got involved in piano and composition in the first place. But the pieces that I wrote at that time weren't that great, to tell the truth. There's so much you can learn about theory and composition, not just from studying theory and composition, but from studying on an instrument. If you study an instrument your music becomes more human, it can connect more, and you learn what instrumentalists are capable of (and are not capable of). Also you learn a great deal about how other composers accomplished what you are trying to.
That's about all I have to say for now.
