Fat is the last (not least) edible part from a slaughtered animal. Fat is actually one of the most filling foods by weight. It's a very good idea to put the fat you collect into your meat soup (it counts as "raw meat" in any recipe), instead of using it for tanning, because the soups come out delicious and very, very filling. Tree barks from Rowan and Alder trees can be used for tanning material instead.