they do help as they eat algae, but iof your tank is planted with REAL plants then the snails are likely to eat them, and also they breed well so you might have a few million snails in a week or two
What kind of snails are they - little and round or cone shaped? Get rid of the round ones, they eat plants, but the conical ones are good for the tank - they burrow into the substrate and aerate it.
Contrary to popular belief, most of the common snails you find in shops, don't eat algae though they will definitely eat your plants. The Malaysian Trumpets are one of the few plant safe snails you'll find. They're the conical shaped ones gadazobe mentioned.