Learning the theory of music is essential to being a good musician. You can play music without it, sure, but having knowledge of the theory behind what you are playing is priceless. It helps you to understand why the composer did what he/she did, why the music all of a sudden has accidentals (key change) or how the phrasing works, for example.
It teaches you the “grammar” of the music you are playing. It’s like learning the grammar of the language that we speak, for indeed, music is a language, a beautiful universal language that people are naturally drawn to. A language that permeates our lives and enriches out existence. Imagine a world without music, no soundtrack to movies, no singing Happy Birthday, no Christmas carols, no karaoke! We sing along to songs in the car, we sing in the shower, we hum to ourselves, we teach the alphabet through music, music is everywhere.
Of course we all know of the countless studies that ALL say how music is so important for our brains and benefit us in numerous ways. The great composers spent years studying theory, as have many popular artists of our times. This is not to say that studying theory makes you a great musician, but it helps to make you a better one. It opens up a myriad on new opportunities for the musician and lets musicians communicate on a deeper musical level than the average person. The more knowledge and experience one has the more it will show in ones music.