We generally don't depend on definitions to understand or use words correctly.
We dio if we wish to talk to each other intelligently.

Ever seen a classical music concert advertised as being classical music? I don't think so. You are meant to have the common sense to work out what genre Monteverdi or Mark-Antony Turnage belong in.

The only genres I can think of that like labelling themselves and seem to benefit from it are jazz and instrumental/electronic dance music. The dance music scene is very fragmented and it probably does help to tell the punters exactly what flavour they'll get; jazz is unusually cohesive and rather easy to label.