No one likes eating the same food for every single meal. Varying your pet's diet exposes them to a range of proteins, flavors. Regularly switching your pet's diet may also help prevent them from developing sensitivities to specific proteins. Plus, it's tasty for them to try new things!
There are also practical reasons for switching foods regularly. If you move, or if the prices change dramatically on your pet's food source, it might be time to shop around for something new. As your pet ages, your vet may recommend switching to age-specific or prescription foods to deal with any medical issues that arise.
It may not be listed on all of our food labels, but there is an ash level in all foods. Learn about ash (and phosphorous) and how it affects your pet.