MusicTheory provides us a way to discover order and meaning in the musical sound and its written representation. Understanding of different pieces, styles and structures require different approaches, either more general or more specific.
Did you know that our modern MusicTheory goes back to Aristoxenus a student of Pythagoras?
http://en.wikipedia.org/wiki/Aristoxenus
See http://en.wikipedia.org/wiki/Music_theory