Humans do not have digestive juices that can digest cellulose still they consume fruit & vegetables that contain large quantities of cellulose
Humans do not have enzymes to digest cellulose as other herbivores do yet they take vegetables and fruits as they form an important source of roughage, vitamins and minerals which are important nutrients. Cellulose forms a part of roughage and it helps in smooth bowling movement of the food and aids in digestion.
Cellulose, the main type of insoluble fibre in the human diet. Cellulose contains just as much energy as starch because both molecules consist of glucose subunits. It is only possible to use that energy by burning wood and other cellulose materials. However, if that energy were physiologically available, humans could lower their food consumption and produce much less digestive waste than they currently do.