Music theory covers the language of music, the grammar and the punctuation. It applies to all instruments, and can be studied without an instrument in mind, as long as you don't mind humming a tune now and then.
Most of my pupils opt to include theory in the lesson plan, to help build on a better foundation.
Often "left behind" by self-taught players, the language of music is no more difficult than learning the instrument itself. In fact it is far easier to relate to written music if you are already playing, at any level.