 If you have like a group of parrotfish and they're all grazing on a reef, if there are no predators, they will just keep grazing in the same spot until they decimate that section of the reef. But if you have predators, that forces them to sort of move around because they don't want to get eaten. So instead of overgrazing on just one spot, they're spreading it around so it's not as detrimental to the habitat.