Why do we slap on horseshoes? Horses got along just fine before we domesticated them…and I would imagine that there is substantial pain to the animal as well.
Yes, they “got along just fine”, but nobody forced them to pull carts along the paved roads.
Nope, no more pain than when you cut your toenails. As to why we do it, it’s for pretty much the same reason people wear shoes when they did so well without them for so long. It makes the feet more hard-wearing, allowing the animal to travel further, faster and with heavier loads without getting the horse equivalents of blisters, corns and broken toenails.