As I'm understanding it, it was primarily for grease, not soup itself. They certainly could have been making soup as well, but grease itself is a much more valuable product. It has utility for waterproofing, warmth (if you smear it on your body) lubricant, and is also highly calorically dense. You can also use it to preserve food (see "Potted meat") and on it's own, stays edible at room temp for a long time.