The snail get it's shell when they are first born.
because they don't have very much shelter so they use their shells to hide in
put your finger inside,twist it,then eat it.
I don't get what you're asking, but I assume you're asking what's it made of. The answer to that is mainly calcium.
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...