Why is it "better" to feed dogs a grain-free diet? - egg free salmon burgers

Here are the ingredients of dry dog food is chicken soup from their website:

Chicken, turkey, chicken meal, ocean fish, cracked pearled barley, beans, rice, oats, millet, rice, chicken fat (preserved with mixed tocopherols), potatoes Earth, egg, tomato pomace, duck, salmon made, flaxseed, natural flavor Chicken, choline chloride, dried chicory roots, kelp, carrots, peas, apples, tomatoes, blueberries, spinach, milk powder, cranberry powder, rosemary extract, parsley flake, yucca schidigera, L-carnitine, dust and Enterococcus faecieum fermentation product, Lactobacillus acidophilus, Lactobacillus casei and Lactobacillus plantarum, dried Trichoderma longibrachiatum fermentation extract, vitamin E, iron proteinate, zinc proteinate, copper proteinate, ferrous sulfate, zinc sulfate, copper sulfate, potassium iodide, thiamine, manganese, manganese oxide, ascorbic acid, vitamin A, biotin, calcium-pa