Music theory doesn't decide what you play, it describes it... love that quote. Personally I've started taking some music theory last two years and I feel it enriches my song writing and giving it an extra dimension.

But maybe most of all it can help me if I get stuck in writing or arranging a song. For a hip hop collab I did where I wanted to have a simple but groovy guitar line I just brought up some scales, and bam... there it was :)