A quick thought on music theory in general: If you have to ask why you need it, you don't need to know about it.

You can write pieces that are modal without knowing what modes are, and you can have a lot of fun with classically trained people by writing a modal piece with a VII-I cadence. I've had people tell me that I'm doing it wrong, which I replied with "No, you're doing it by the book as opposed to by ear".