Music theory is the theory of how music works. In other words, music already works without theory. But theory is useful for understanding what made that amazing music you just heard. Theory is broad enough to include any statement, belief, or conception about music. In other words, theory is how someone might analyze why things sound good, but is not the end all to playing well. Music theory is observation. Music came before theory. Music comes from within.
Theory can really come in handy when you’re looking to play something complex. With high caliber musicians, theory can help to make sure that what you have to say fits in with the rest of the conversation. It’s like trying to have a conversation with a handful of rocket scientists. You’ll only be able to say so much if you’ve only mopped the floors in the lab.