Why do some invertebrates have exoskeletons?

The joints in the exoskeleton, evident in the appendages and body segmentation, are essential because of the physical rigidity of the exoskeleton's makeup. It is constructed of a tough protein, chitin (similar in function to hair and fingernails, claws, beaks, and so forth) and typically further hardened with biomineralization. Because of its limited flexibility, a strategy would have to be employed to provide limb and body flexion, for example, to facilitate locomotion; walking, swimming, flight. Without limb jointing, the organism would be immobile.