There are a number of physiological reasons for being advised to eat foods high in fiber such as soluble or insoluble fibers. However, the main reason for eating those foods, even when they contain primarily cellulose, is because we can't digest it so it thickens our fecal matter. This provides bulk to our fecal waste so our digestive tract is able to move the feces easier with little strain. A strenuously working digestive tract is common in people who lack fiber resulting in constipation which could lead to medical complications like diverticulitis, anal fissures, hemorrhoids etc. It is important to have a the right balance in fiber so the feces are not too hard nor too soft but just right; salads with little nutritional benefit (in terms of vitamins and minerals) are advised when they may provide needed bulk for our feces.
