They have them for a clean up crew, because they like them but most become a nuisance since they reproduce like CRAZY so don't get any without research, they also are hithikers on live plants so it's good to give them can't remember but a type of bath
There are many different types of snails. Some are good, and some are pests. Most folks have the big snails like mystery snails and malaysian trumpet snails because they are good clean up crews. The mystery snails are good for algae, and the MTS eat dead plant matter and leftover food on the bottom. They also burrow in the sand, so they keep it stirred up which is necessary to keep from getting deadly gas pockets. Pond snails usually hitchhike on live plants introduced into the tank and become pests when they overpopulate the tank.
Ugh! I had a few come in on a Cardinal plant that I bought. They burrowed into the root ball and now I'm plucking them out 3-4 at a time!!! It started as one, then two....now, I've taken at least a 1/2 dozen to the LFS and they keep creeping up!!