Asked by: Graham Needham, by email
There is no 'point' to slugs, any more than there's a point to lions, mosquitoes or humans. Organisms do not exist to serve a role as part of some higher purpose; they exist because they are able to. It's the property of life to expand and diversify to fill all available ecological niches. Slugs are capable of much wider colonisation than snails because they don't depend on a source of calcium to build their shells. They cause a lot of damage to garden plants and crops, but they also help to clear away rotting vegetation and are themselves a valuable source of food for toads, slow-worms, beetles and birds.