The whole reason for having genres is too help classify and group together music/musicians. If I want some recs for melodeath bands like Kalmah and early In Flames, I'm not going to appreciate somebody saying Gorguts, Belphegor, or Prostitute Disfigurement, because those bands aren't what I'm looking for. Not all Death Metal is the same, so having subgenres makes sense. You don't need to get all faggy with things like "Atmospheric Technical Dancecore" or whatever the fuck, but simple subgenres like "Blackened Death Metal" or "Brutal Death Metal" are necessary.