theres also some dungeons and raids where a mob will cast some sort of shield, that absorbs lots of damage, like in Naxx the military wing, some mobs cast bone shield which effectively kills all DPS while its up, and must be removed. I just spam my priests mass dispel when that happens, but purge should get it as well. Too bad they nerfed mages spellsteal on the bone shield, after that one mage soloed razuvious in 25 man with that buff... Also in Utgarde Keep the runecasters cast a shield, I usually burn em down and stunlock em before it casts, but its nice to dispell it when it happens.