For people whose goal is sustainable ag on a small farm, and getting waste to the absolute minimum, the pig is the final stop in keeping nutrients and energy at home.
You grow it (crops), you eat it + feed it to the chickens or sheep or turkeys or whatever, <insert dogs and cats here>, you compost plant-based leftovers and eggshells and shredded paper to feed the soil, and whatever’s left that has absolutely zero other takers (meat scraps, bones, used cooking oils, really awful anything else), it goes to the hog. Who then gives back at slaughter time. Aslop perfect circle.