How effective is the protection offered by a snail's shell?

They are not that much use against a larger vertebrate predator, but they can certainly protect against insects and others. Even then, it is not perfect, because as with other creatures, there is an "arms race" between prey and predator. Even for vertebrates, the shell does provide some protection. read more

And snails seem to be a "second best" food for most vertebrates that eat them. Perhaps the added fuss of breaking the shell is a turn off. But the shell is a good protection against drying out, and white shells can reflect quite a lot of heat from the sun's rays. read more