When a Pokemon's job isn't to kill or maim something but merely to harass a specific target. Another example I can immediately think of is tank chomp. It usually has toxic, rocks, equake and fire blast or endure with jolly nature. It's job is not to deal damage but rather to spread toxic and it uses fire blast to scorch Ferrothorn and other steels because they can't be poisoned. It's never gonna kill anything but at least it makes the durian think twice about trying to eat a toxic
That sample Silvally set is probably meant to harass opposing steel types since they wall this set and there are three waters in current ss pu, which steel Silvally can bait in and switch out with parting shot
And like sumwun said, utility moves also don't really mind the minus nature that much since they're not meant to deal damage anyway. This is why someone like Tapu Koko runs timid with you turn because it switches out anyway
In short, utility moves or just to harass a specific target don't mind the minus nature