Atmas allow a different testing environment, before abyssea came out there weren't a load of jobs that can solo VT and IT mobs. Now you can do that with any job. Try AGI atmas, regen atmas, DD atmas, a blend of the 3 to see how guarding MNK differs from counterstance monk, damage wise and defense wise. Slightly easier to test in abyssea when you have atmas to keep you alive, than outside of abyssea where you would need to be PLed continually.