when are fire and magic damage actually useful for armies?

Topic. Asides from the occasional hero or unit with physical resistance or regeneration, in which situations are fire / magic damage genuinely really useful?

The big obvious one is chaos; all(?) the slaanesh and demon units have physical resistance which really adds up given how quickly they close to melee. So yeah fire or magic damage is almost essential here.

But most of the rest of the time, they don't feel that impactful on an army level?

Tomb kings as a whole don't really have weakness to fire (?), neither do vampires or wood elves unless you're fighting Drycha or Durthu.