Why does the snail have a shell?

An educated guess of mine is adaption. Yes, there are slugs, but they are more prone to be eaten or attacked by predators because they don't have a source of protection. Their vulnerable, slimy skin is exposed completely. Maybe snails are an adaption of these slugs, a twitch that Mother Nature made, by adding a shell. So when a snail is attacked, all they have to do is slip into their shell and they are probably nice and safe (unless the predator is smart enough to flip the snail over and peck at the flesh inside).