Categorising Music Into Genres

We looked at all the diffent sub-genres that have sprung from the genre Heavy Metal. In total I found over 50 different sub-genres. This raises the question over why we need these genres and who catergorises bands into them. Is there a need for our music to be catergorised?

We as a class felt that bands or record labels actually promote sub genres so a band's style appears original and different. In fact alot of the bands actually still crossover the barriers of these sub genres, which was highlighted when someone felt Rage Against The Machine were industrial metal, whereas many others classed them as political rap metal with funk elements.

Wikipedia classes RATM as "Rap metal, funk metal, rap rock, alternative metal". This is infact correct as no electronic sounds are used, the band's album booklets always state "All sounds on this album are created by use of vocals, guitar, bass and drums only".