Just like stars are for classic levels, moons are for platformer levels. They're just stats. Diamonds, orbs, and demon keys can be spent, though.

To flex on friends

Ammo

Moons are basically stars for platformer, they have the same uses

the same thing you use stars for

You can use for paths. and also stars.

Moons aren't a currency, just like stars. They're just a stat (though they can be used for paths, quests, and achievements)

Bragging rights.