Heels were useful for cavalry in real life - alongside bulky leg armour - since the horse does all the moving for you, and if you're dehorsed you're basically dead, that basically leaves your legs with two purposes - spurring your horse, and attacking your enemies when they're not in position for your gigantic spear. (or too close to shoot with your bow, or shoot reliably with your flintlock, or whatever) A high heel will help that kick become a stabby or slashy kick. Sure, it won't be useful often, but there's no real drawback.
For foot soldiers, on the other hand? Best suited for settings where there are dragons and spells and things.