Nobody becomes a composer out of a need to express himself or out of a wish to affect the course of music history. We all begin by falling in love with a piece and wanting to make something just like it. With the passage of years, the urge to imitate disappears, but most of us continue to be nourished by the study of music.
The importance of theory is obvious. You cannot really understand Bach unless you have internalized the syntax of functional harmony along with the principles of long-range voice-leading. And the various techniques of extended tonality, free atonality, or serial construction are equally important for music of the 20th century. But no way of thinking about pitch or rhythm provides an automatic advantage; theoretical constructs are only useful as a source of opportunities. What interests us in a piece we care about is the particular occasion, how technical procedures are placed in the service of a musical narrative that is uniquely compelling.