Prevent cancer. High fiber diets, prevent colorectal cancer, and help people with constipation. The fiber enables you to have more frequent and softer bowel movements. Toxins and contaminants in food, don't stay as long in the colon as those who are not on high fiber diets, so there is less exposure to cancer causing toxins.
Many benefits. I describe dietary fiber to be like eating microscopic sponges. These "sponges" absorb water (keeping the stools soft, reducing constipation and reducing colon cancer), and absorb dietary fat (reducing cholesterol and triglycerides, thus reducing heart disease.).