@talas @nfzeta: Probably because it's handy. It's not the same watching a fight between a knight and a dragon that between a knight Lv. 15 and a Dragon Lv. 50. You get a measure of the power levels and that would make things more interesting. To me it really becomes annoying when it gets too ridiculous (Kumoko or The New Gate) or too RPVG-like.
...Or probably because the avetage otaku has grown up with Dragon Quest, Final Fantasy and the like and are pandering to them (or they are the ones writing), idk. It also happens in western fantasy that looks suspiciously like the Forgotten Realms, Dominaria or Arda.