You can earn the skins in the game, those are if you want them imediately

With their retail priced games this isn’t there normal mo. Starcraft 2 has not had any monetizations other than expansions, and with Diablo 3 Blizzard went to great trouble to get rid of it’s microtransactions. Hearthstone and Heroes of the Storm are both F2P, WoW is their only non F2P game with microtransactions.