Why does spell need mana?

[WC3Wonderer]

1. Archmage has 200 mana.
2. Archmage summon water elemental for 100 mana.
3. Archmage needs more water elementals.
4. Archmage wait 15 seconds because spell is on cooldown.
5. Archmage summon water elemental for 100 mana.
6. Archmage needs more water elementals.
7. Archmage wait 15 seconds because spell is on cooldown.
8. Archmage can't summon water elemental because he has 50 mana.
9. Archmage wait 30 seconds to restore 50 mana.

Archmage attacks every two seconds because basic attack has cooldown. Archmage doesn't spend gold to attack. Archmage doesn't spend health to attack. Archmage doesn't spend mana to attack. Spell is also a attack. Why does Archmage spend mana to cast spell (to attack)?

 

[Tasyen]

Mana is a balancing tool to limit the total spellcasting of an unit. While cooldown adds a pause before you can do the same thing again.

The makers of warcraft 3 melee game wanted to limit the amount of blizzards & Elementals archmage can use in a specific time frame. This was most likely inspired by older rpgs in which casters can use spells only some amount of times per day. In the same time they wanted that the caster hero without mana can do something. Therefore archmage can auto attack for free.

Edit: Spells do something supernatural which shifts the power balance and that is something that blizzard wanted to limit. Only 2 elementals or 4 blizzards for 300 mana.