The snails do eat algae, while the other critters are primarily scavengers. They will eat left over food, dead critters, some eat algae, etc. They are a good addition because they help keep the tank in good shape on a daily basis - they help especially if/when you overfeed, have something die without you knowing it, etc..
Kind of a safetly net of sorts. I'd wait to add the snails until after your cycle is finished. In most new tanks, once the levels are 0, it's never to early to start fighting algae. I'd wait to add the hermits until after you have a fish or two (or more) in the tank... that way they will have something to scavenge - leftovers.