both sclerenchyma and collenchyma give mechanical strength and flexibility to a plant then why do we need both?

The role of both sclerenchyma and collenchyma are almost same but they differ in their structure and also they function in different parts of the plant. Collenchyma give mechanical support to growing parts of plant such as shoots and leaf petioles and their wall is nonlignified. Sclerenchyma give machanical support to nongrowing parts of plant and die after maturity is attained and their wall usually contain lignin.