This thread is perfect for me, because for many years, I was a music cataloger (librarian). (Yes, Dewey Decimal System.) I always felt the internal tension between the artist in me ("I don't believe in categories, man. Don't put me into a box!") and my professional duty to help library patrons find what they were looking for, which absolutely required classification. Actually, I enjoyed the fact that the non-musicians I worked with sort of considered me a kind of savant who was seemingly aware all of the musicians in the universe and the bags to put them in (including classical music and world music).
So, it's complicated... art is subjective, yet there
can be a utilitarian reason for observing the differences between Iron Maiden and Louis Armstrong.

Also, a couple of people have mentioned browsing in record stores which have everything shelved by artist, not separating them by genre. That's great, but in a library setting you also have online catalogs which people search by subject headings, which can really narrow a search. This is a useful, good thing, if you want (say) various 1970s popular music from South Africa, or (say) an assortment of arrangements of Beatles songs, and you don't want to spend half a day plodding end-to-end through shelves which are organized like the aforementioned record stores' bins.