Basically, sub-genres exist only for the purpose of making it easier for music geeks to talk to each other. Trying to explain them to anyone outside the loop is pointless (which is obvious to anyone who has ever tried >.>).

However, between music nerds, sub-genres are an incredible help to anyone trying to properly describe a new band he/she has found. They provide implicit comparisons to other bands, and they do this quickly and efficiently... Of course, only to people in the loop. Again, people trying to explain sub-genres to the average passerby are pompous fools.