What is the biological meaning of such a phenomenon in crustaceans like molting?

All crustaceans have a very hard shell that protects them from natural enemies. The shell is formed from the dying top layer of the skin, containing a substance – chitin, which just gives strength to the shell. Since the shell has a very hard structure, it cannot grow with the body of the animal, and over time it becomes cramped for the crustacean. In connection with this phenomenon, the process of discarding the old and the shell takes place, followed by the growth of the new. Usually, the crustacean moults once a year.