Snails are a double edged thing in your tank.  One one side they eat algae, alot of the waste in the tank, and can be an interesting live food source for many fish.  On the other side they eat alot of delicate plants, depending on where they came from may carry disease, and depending on species and size may reproduce rapidly and cause an increase in waste in the tank due to excrement and may be hidden away when they die causing a large source of contaminant in the tank while they rot stressing your fish due to ammonia spikes.

So not all snails are bad though.  In many tanks specific species are desired for their cleanup ability and decorative patterns and shapes.