[TheRapingDragon]

I've always been a little baffled by how many "sub-genres" of music there are, and why anyone in particular cares whether a song/band is progressive or alternative or avant-garde whatever >_>

The only reason I can think of is when trying to find similar bands to the one you like.

If all metal was classed as "metal" then if you enjoyed Dream Theater someone could recommend Bloodbath as a similar artist you may enjoy, they're both metal after all. By having genres you can break down specifically what you enjoy and find similar bands quicker.