Tank Mitigation vs Tank DPS.

[Raven2014]

Raiding is like a "the poor get poorer" and "the rich get richer" things, with DPS as the main currency. If you don't have enough DPS, the fight get longer and that means you have to deal with more and harder mechanic which usually stack at the end of the fight. Group with high DPS can clear the fights without even seeing the "hardest" part of it.

This also applies to outside of raid, like dungeon. Things die faster = tank takes less damage = healer has to heal less.