A lot of traditional cooking and recipes are based on
systems. They interconnect with other cooking. So you make one dish or you slaughter one animal or whatever, end up with fat or lard or something as a byproduct, use that to make other dishes. A bunch of stuff ends up in soups or stews, which you don't so much make
intentionally as
as an accident of other cooking, to stretch out ingredients and use up parts that otherwise aren't so great.
These don't make sense anymore when we don't really stick to regional, seasonal, supply-dependent cooking and can decide to make any damn thing we like at any time, thanks to grocery stores, and simply toss most of the extra bits ("useless" parts of vegetables, bacon fat, et c.) if we don't feel like using it, because the time, space, and technique it takes to use them correctly's not worth it when we can buy a substitute from the store for very little money. Quite likely a well-proportioned substitute of butter for lard doesn't make much difference in the outcome of that recipe, but there was originally a supply-related or place-it-fits-in-a-cuisine-system reason that lard was preferred.
[EDIT] to be clear, the old way of cooking is probably still cheaper in general and certainly less wasteful, but the savings are so marginal relative to the skills, time, and space required that it's totally reasonable that even poorer folks largely don't bother. This kind of thing went beyond just cooking patterns and cuisines, too—animals commonly kept by anyone living even sort-of in the country had a role. Chickens? They'll eat waste, control pest insects, give you eggs, and if you're also breeding them you can slaughter one for food every now and then—the eggs and meat are damn near free, as far as dollars put in past an initial capital investment and some trivial upkeep are concerned, though they do take time & care. Pigs? Even better waste disposals than the chickens, turn garbage into meat and cooking fat. These got much cheaper and more common, but also were removed from their role in the broader food & home-economics ecosystem, when we started farming them on a mass scale and mostly feeding them stuff we grew specifically to feed them.