They will eat dead plant and animal materials, thus cleaning the tank. They do not harm live, mobile fish. They are interesting in their own right and part of the ecosystem. If they multiply too rapidly, they could die off. Their corpses would be a major pollutant and could poison other animals in the tank or pond.
Large snails can create copious amounts of waste adding to the nutrient load.
Snails may eat fish eggs or newborn fry. Some specimens may harbor parasites and other ailments which could be passed on to fish. Feeding Snails will usually find enough left over fish food, algae, and live plant material to eat. Only provide as much as they will eat over a day. Each day, remove uneaten portions.