Three functions of an arthropod's exoskeleton are protection, support and muscle attachment. The exoskeleton starts as procuticle, made up of layers that include chitin microfibers in a protein matrix. After each molt, parts of the procuticle harden, forming the familiar exoskeleton of a beetle, centipede or lobster.
A hard shell protects the arthropod's interior organs from damage by predators. It supports and shapes the body. The interior of the shell provides attachment points for skeletal muscles. This musculature give optimum leverage to the appendages and provides the incredible strength seen in insects. The strength and protection come at a price: an arthropod cannot continue growing while encased in a hard shell. Every so often it must molt, shedding the old shell, and remains vulnerable until the new shell hardens.