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. Rotating foods, importantly the contents of those foods, every so often helps to give them the variety their bodies were designed to thrive on.
_ There is no one dog food in existence that “has it all”. And remember, the feeding trials that foods go through only last for six months (and not all foods are examined the same way). Rotating foods periodically helps to ensure that no dietary deficiencies or excesses build up over time.
_ Variety is the spice of life. Who wants to eat the same food day in and day out? Could you do it? For months or years? Feeding your dog something different helps to ensure that he does not become bored and frustrated with his food. This will not make your dog picky - we are talking about periodic changes here, not serving up something different or adding goodies to tempt him every time he doesn’t like his dinner.
And most importantly of all:
_ It helps to avoid the development of allergies. In a few extreme cases, you (or your dog) may be instantly and violently allergic to something. But that is a few extreme cases only - the vast majority of allergies are things that build up over time and with constant exposure. The surest way to develop an allergy to chicken, for example, is to consume it daily for an extended period. It is no coincidence that the most common allergens are things that have commonly been used in dog foods for many years. The (modern) advice given by nutritionists is that feeding a wide variety of different foods, preferably from a young age, can help to avoid the development of allergies in the first place (that's the advice given for humans too).
Common signs of allergies are itchy skin, red itchy paws, chewing paws, yeast infections, ear infections, and skin infections that may respond to antibiotics but reappear as soon as the antibiotics are discontinued.
That all adds up to rotating the food you feed every once in a while. That means rotatinging to a food with different main ingredients. There is very little benefit to switching from one chicken/rice food to another, for example. If you have been feeding a food with chicken as the main ingredient, then it is far better that the next food is based on lamb, or turkey, or fish, or beef, etc and that the other main ingredients are also varied.