Spirits of Kin are this edition's Spirits of Man. They in turn are the successor to both City Spirits and Hearth Spirits of much earlier editions.
Basically lore/fluff-wise, they're the game adaptation of gnomes, faeries, knockers, leprechauns, brownies, etc. As well as, from animist point of view, the essential spirits of city busses, vending machines, alleyways, and so on.
As for what the different kinds of spirits are "good" for... mox nix. The critter powers you select count for more than the type of spirit.
