Why does Jarvan use mana? His moves are throwing a flag, sticking out his spear, and jumping really hard onto an enemy.
Why does Miss Fortune use it? Her moves are shooting bullets, shooting poison bullets, shooting a lot of bullets, and shootings a LOT of bullets.
Why does Graves use mana? His moves are shooting bullets, throwing a smoke bomb, moving really fast, and shooting a wide spread of bullets.
Etc, etc.