They mostly need them to wear down their teeth. Puffers crunch the shells to get at the snails, this grinds their teeth down like it normally would in the wild. IF you don't have snails or some other crustacean for them to feed on, their teeth will continue to grow until they will no longer be able to eat or close their mouth properly. From all I know, that's why its important to feed your puffers snails or ghost shrimp or whatever.