i think maybe the benefit of the fish is that their tiny mouths go everywhere, not just the flat areas you can use your pumice stone on. kind of like that liquid callus remover some salons use. they just paint it on so it's more 'precise' than a pumice stone and it can also dissolve the hard skin around your nails where you wouldn't use your stone. plus all the rest of your skin.
basically, i think the fish kind of do the same thing as a paraffin treatment or a good exfoliant, except it's more fun cause it's live fish eating your feet