Yes and No!
I honestly don't know a lot about snails so some of the more experienced snail keepers on here will need to respond, but there are some in Spot's tank and I think they are a good thing. They are tiny little black snails. I think Spot eats them and they are a source of calcium and entertainment. I find some in his filter but I just wash them out and they haven't caused a problem.
Some people have had snail population explosions and it's been a real mess. The snails leave icky egg slime around and clog up the filter.
I don't know why sometimes snail populations get out of hand and sometimes they don't. Maybe there are different types of snails.
Sometimes people buy big pretty snails for their tanks but most often snails come into tanks as stowaways on water plants. You can prevent them from coming in this way by washing the plants very well before putting them in your tank. Once you get them in your tank they're really hard to get rid of if you don't want them.