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The orc needs magic to suppress enemy spells; he has an ability for that.
The ability "Strength against Magic": Weakens all enemy spells: when calculating their effects, the enemy's magic is reduced by 1/2 of the hero's magic.
If it didn't cost mana, it could be used unlimitedly, but at least there's some limitation now.
Essentially, the fact that mana is required to use spells is what connects it to magic, but otherwise, it's not magic in itself.
It's just so the hero doesn't get bored on the battlefield and constantly attacks enemies.