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 nutrients, 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.
Sp, in short, even though cellulose cannot be digested by humans, cellulose helps in better suggestion and excretion.