A couple of months ago Bug Safari posted some images of a slug eating a snail shell.
As any of you who keep snails as pets know, snails require a source of calcium in the diet in order to grow their shells. In captivity this id usually provided by giving the snails cuttlefish 'bone' or powdered chalk in addition to their usual diet. In the wild snails mainly get their calcium requirements from calcium slats in the soil. This explains the relatively higher concentration of slugs (compared to snails) in areas with low soil calcium.
Some slugs do have crystalline calcium carbonate in their mantle, as shown in this photo from Snail's Tales.
Eating snail shells is a great calcium source for slugs which have calcium carbonate in the mantle.