Carrots are a relatively new ingredient being used in pet food but are generally recognized as safe (GRAS; 21CFR582).
We use Carrots in small amounts (less than 2% of the diet) in many of our recipes because there is no scientific evidence that a dog or cat is or can be allergic or sensitive to Carrots. There is also evidence that suggests carrots enhance the palatability of pet foods. Carrots also provide a great source of nutrients & appropriate amounts of fiber for our companion animals to promote optimal balanced health & nutrition.
Carrots also DO NOT have a substantial source of concentrated protein like many vegetables (especially potatoes & legumes), allowing us to accomplish a complete and balanced diet with the protein source coming from quality meat, poultry & fish proteins, not vegetable proteins which are typically high in sugar & carbs and are used by many pet food companies as a cheap alternatives to animal protein.
Carrots also provide a tremendous source of soluble fiber for our pets and are rich in natural potassium, alpha and beta-carotene, and Vitamin B6. There is some evidence that this helps with immune functions in dogs & cats. Swanson et al., 2001 found generous levels of butyrate which is beneficial to the colonocyte for gut health.
Carrots are not a cheap ingredient like many vegetables included in pet foods and should not be considered a "filler." We believe this ingredient has many beneficial attributes with few if any recognized drawbacks and should be recognized as a positive functional ingredient in our high meat, poultry & fish recipes.