Answer:
They are building blocks for larger polysaccharide which are important in structural rigidity of cells.
Polysaccharides such as Starch and Cellulose which are instrumental in cell structure and rigidity are formed from chains of monosaccharides.
They are also useful in the provision of energy which is used in cell functions (metabolism).
Monosaccharides such as Glucose, Sucrose are useful in provision of energy for physical and metabolic activities of the body.