One of the reasons pet food manufacturers add dietary fiber to processed pet foods is to supply prebiotics, which can aid in the growth of friendly bacteria in the large intestine of dogs and cats.
According to PetfoodIndustry.com:
"Prebiotics are food for probiotic, live microbial species that are capable of colonizing the intestine once ingested. Due to myriad issues, probiotics often struggle to colonize and remain colonized in the intestine.
One method to overcome this problem is to use synbiotics, a combination of prebiotics and probiotics, which increases the persistence of the bacteria in the intestinal tract. Persistence is improved by providing a preferred food for the probiotic, thus promoting bacterial growth and enhancing colonization."
Another reason dietary fiber is added to pet food is to increase satiety (a feeling of fullness) in dogs and cats, which helps with weight loss programs for overweight and obese animals.