Theory is good for understanding music.
Well, the right theory for the right music. Roman Numeral analysis isn’t so great for understanding contemporary pop. Jazz theory isn’t great for Renaissance motets. Counterpoint is a great tool to clarify some aspects of much music, but if you’re trying to understand an orchestral film score with it, you’re going to come up short.
Understand, then, what the theory is going to do for you…and also what it isn’t.
Theory isn’t good for creating great new compositions. Harmony, certainly, is valuable. But so is melody – something traditional music theory barely touches on. Form is often touched on for a much briefer time than 4-part Chorale-style writing…but an understanding of form is going to be a much more important tool for an aspiring composer.