"Fiber refers to a multitude of compounds categorized as complex carbohydrates. Fibers differ from starches in that fibers resist enzymatic digestion in the small intestine. As a result, fibers are usually fermented by microbes in the colon... The primary function and benefit of adequate dietary fiber are to increase bulk and water in the intestinal contents. Fiber appears to shorten intestinal transit rate in dogs with normal or slow transit time and prolongs it in dogs with rapid transit rate. Together, these fators help to promote and regulate normal bowel function. In addition, the typical end products of microbial fermentation of fiber (acetate, propionate, and butyrate) are important in maintaining the health of the colon... The antibacterial properties of short-chain fatty acids may decrease pathogenic intestinal bacteria, increase resistance of the gut to colonization by pathogenic bacteria, and may be important in prevention of and recovery from intestinal disorders and cancer...
... Fiber is not considered essential in the diets of cats and dogs, although it is often included in commercial foods. Overall, dogs and cats do not derive much energy from absorbing the typical end-products of bacterial fermentation; however, short-chain fatty acids are important in maintaining the health of the colon. Therefore, a small amount of fiber (less than 5%) that contains both rapidly and slowly fermentable fibers is recommended in foods for healthy pets."