Eater is a fan-shaped blast in front of Shulk that deals decent dmg (not as good as Buster tho), inflicts Bleed, and clears all buffs the enemy/enemies has/have. It's kinda like an in-between option of Buster (raw dmg) and Purge (clears buffs, but doesn't seal Auras). It's pretty good in certain situations, but it's by no means essential. Armour is a flat increase to Def & Eth Def for the party.