Many times Snails hitch a ride with plants from the fish shop. Some carry parasites, but there isn't an easy way of detecting them. If your fish and tanks are healthy, you should be okay most cases
I have a recent infestation of surprise snails. I remove them and more often the fish eat them.
A few unintentioned snails can quickly turn into a large out of control infestation that compete for tank resources and emit that much more waste to pollute your
tank.
I buy Neerite Snails because they work hard to eliminate algae and detritus from the tank. They are attractively colored additions and can't reproduce in most tank conditions, requiring special brackish water conditions.
You can tell if your snails are reproducing if you see flat, clear circles against your tank walls. If you look closely, you will see tiny clear eggs inside the gelantinous masses. I remove them. Snails may also eat your plants, although Neerite usually don't.
Cichlids, puffers, some loaches-yoyo loaches and other fish will eat Snails. Crush the snail shell against the tank wall, and most of your other fish can pick out the meat as a treat.