Why does Fel Armor exist?

I've been wondering lately why exactly fel armor was put into the game, it just seems like a flat buff to warlocks for no reason, 20% more healing (basically passive) seems a bit extreme, especially to the class that already has almost the highest self-survivability in the game at the moment.

So what do you think? Why is warlock the only class (WITHOUT TALENTS) with a flat +20% healing from ALL sources? (don't forget the 100 free spell damage).

Edit: Yes I know it is dispellable, purgable, but it costs like 500 mana to recast? Warlocks have everlasting mana remember?