This is also from the Dog Food Analysis site, which I mistakenly posted under general discussion. A brief essay on good reasons to vary our choice of dog food, ensuring they get not only variety but also a good mix of ingredients and supplements.
Dogs (actually all mammals) are designed to consume a range of different foods, and to obtain differing vitamins and minerals from each. They are not designed to eat “only chicken” or “only lamb” or any other food item for eternity. Changing foods, importantly the contents of those foods, every so often helps to give them the variety their bodies were designed to thrive on.
Here's the link to the dog food analysis/reviews: