Probably for the players who want to play through the story and level jobs on roles they might not be good at (tank/healer) solo.

Their damage is so low that it is mind numbingly slow. (avg 25+ minutes depending on role) But that is done on purpose. If they killed near actual player speed people would stop queueing for them in the DF since their mechanic failure rate is pretty low already in my experience and you get a decent amount of loot.