It's all about how music is promoted and sold. The record companies e.g. presumably have 'expert' rock people in their rock division who know the profiles of the people who buy rock and know what would sell and what wouldn't, and they know how to market to them. Same for any other type of music. If they didn't categorise music their marketing would have no order. I have no experience of online music buying but record shops needed to categorise their music so that rap fans and ambient fans alike could FIND their favourite category of music to look through and maybe buy somethng. If you had no genres none of what I've described could happen properly.

 

yes I know, I was being ironic

 

you get it in the cinema... like ice cream.