If you are raising animals as pets, do you want them to have the best nutrition possible?
Even if neither of these bother you, would you rather have healthy animals, and prevent illness by good feeding practices, instead of having to spend money on medicines, and replacement stock?
For me "all of the above" applies, and another factor, frequently overlooked. Meat raisers are blanketly accused by animal rights groups of cruelty. On my farm every animal, no matter what it's ultimate fate, is cared for as humanely as possible, in fact they are spoiled! There are many corners I could cut to save costs, but it would be at the expense of their welfare. I don't NEED to give the pigs the huge space they have to run around in, but I love to see them play. My rabbits have toys. The chickens are petted daily. I once stood and cooked macaroni and cheese from scratch as a treat for the pigs. Right now we have a commercial meat chicken who will never see the butcher. His legs gave way and he has a box of his own, and is being fed an expensive diet to keep him alive and pain-free. There is LOVE in correct nutrition.
See also Farming and Spirituality
Of course the question remains, what is the best thing to feed your animals?
The obvious thing to do is to try to mimic what the animal would eat in the wild. But bear in mind that many domesticated animals have developed quite differently to their wild cousins, and their needs have changed accordingly. The purpose must also be considered. A goose will survive by grazing, and never being "fed". But if you want him on the table, it makes sense to give him grain. As soon as an animal is restricted in his grazing, we must provide. One way to do this is to have pasture that includes a wide variety of herbs. Land not treated with pesticides will usually provide without assistance (thank you birds!) or you can help things along:)
Consider the digestive system of each animal. Mineral requirements can vary enormously. Trace elements vital to some species are toxic to others in any quantity. This can be a challenge to the home-steader with a traditional "Barnyard" arrangement. Some facility will be needed to ensure these specialist items are available only to those it is intended for.