@RickJameswitit 2 reasons. At 100 corruption, you grow a tentacle on your back that randomly throws shadowbolts at raid members, which do a lot of dmg. When you iceblock, it gets rid of the tentacle forever (which is why mage is the best choice to get 100 corruption over other classes). Also, when he eats a shadowlord, he casts 3 waves of shadow nova, and since you can't get healed, I try to avoid as much dmg as possible. So blocking the first shadowlord he eats avoids 3 shadow novas.
@keridwan It's part of the fight. When you reach 100% corruption all your spells do double dmg and are instant cast. Mages get 100% corruption on purpose for the dps needed to kill the boss.