Pet food companies use fillers to make more profit on pet foods.
The term "filler" is used to vilify ingredients used in pet foods by competing companies. There is no exact definition for this term, however, it often points at sources of fibers or carbohydrates in the ingredient list. Fibers can confer health benefits for the pet including supporting gastrointestinal bacterial balance, motility, microflora and maintaining stool quality. Certain protein ingredients have also been implicated as "fillers" such as corn gluten meal. These ingredients often provide concentrated sources of protein and energy as well as fatty acids and vitamins that are highly digestible for the pet. Corn gluten is used to provide essential sulfur containing amino acids which are deficient in animal meat but needed by the dog and cat. Careful thought is put into each ingredient selection in a pet food for there is no one perfect ingredient. It takes many, each fulfilling a purpose, to meet the current recommended nutrient requirements of cats and dogs.
If you have more specific questions - Ask the Nutritionist.