It's simple, in the beginning theory is what you need to know to be able to read the music. That way you can become much more flexible, knowledgeable and be able to figure things out for yourself and be independent. It also makes talking about your music with others much easier. Instead of saying that wiggly quiet thing somewhere towards the end, you can say the rest at the end of the 2nd phrase. You just sound more intelligent, like a serious musician and the other musicians actual know what you are talking about.
Later on, music theory becomes what you need to know so that you can better Understand the music. Take it from just notes to real music. It will help you learn the music much more quickly and accurately, give you a reference to see if you forget something, and if you are playing music by other musicians, it will help you create a performance that is true to the composers intentions, Then you can put your own spin on it.