Grain free diets avoid allergies.
Allergies to pet food ingredients are less common than environmental allergies in dogs and cats. The most common source of allergens in pet foods is the protein ingredients rather than the carbohydrate although a lower incidence is reported for corn, wheat, or soy proteins. Even for these pets, avoidance of ALL grains is not necessary, rather just avoiding the one offending grain protein would be indicated. Grain free marketing has been a highly successful ploy for the pet food industry but does not provide any nutritional advantage.
If you have more specific questions - Ask the Nutritionist.