Cleanse - Purify all negative effects that are Minecraft debuffs (slowness, poison, wither, blind, etc) that are on you.
As for why, I think it would make some fights more engaging in interesting ways. Bind your Cleanse spell to an item, keep it on your hotbar, and you've got a good reason to cycle items on your hotbar mid fight. Currently I sit on my bow unless I'm fighting an Enderman (or Lesser Ghast) or am hurt/hungry. I like reactive use spells because they make combat more complicated and add intelligent decisions you have to make, which is interesting.
That said, if there are