Your granddad was smart, and the reason you are looking for is quite simple: dogs are
carnivores and therefore designed to function their best when eating animal flesh, bones, and fats. The bacon grease was simply a pure fat source, which is preferable to any carb source.
Most people believe that dogs are "supposed" to eat kibbled feeds, but this is simply not so. Kibbled feeds were designed for
human convenience, not canine nutrition. There is no "kibbled" form of dog food that will ever be a substitute for they flesh, bones, and fats dogs were designed by Nature to eat.
A lot of people don't realize
dogs have no need for carbohydrates. Their digestive system is designed to process fats and oils for energy, not carbohydrates. Dogs can't even process carbs without cooking. The carbs you feed them would come out looking the same as they went in, without cooking. In fact, the entire "kibbling" process that is used to make store-bought kibbled dogfood extrudes and heats the kibble to unbearable high temperatures ... just so a dog can actually use the carbohydrates (corns, wheats, rice, etc.) ... but in so doing it destroys whatever meat content was also in there, ruining the natural enzymes, and taking most of the food value out of the product in this very "kibbling" process that makes it. This is why dogs fed a proper raw diet will always look (and be) healthier than dogs fed kibble. Feeding dogs kibble is like raising kids on McDonald's french fries: overprocessed junk food compared to a proper diet.
In the Alaskan Ididarod races, for example, almost all these competitors feed their huskies are huge amounts of raw fats and meats (mostly fats) with NO kibble and NO carbs. None. Your grandpaw was simply giving the dogs a fat source, not a carb source, and dogs simply do better and look better fed meats and fats, not carbs
