One rock hurts a lot, so it's safe to guss that many rocks hurt a lot more.
— Game Description
The Catapult Volley is a very powerfull skill, is ia s bunch of rocks fired against the deadities. The same rocks the Wall Catapults use are thrown in this skill. You might want to use it when you are being attacked by Deaditie Archers or Spear Deadities, which have low health and die easily with that skill.
This skill is one of the more powerful skills in the game, that is why it is the second last one to unlock.