It is a body cavity (or space) on addition to the gut/digestive system.
what benfit do animals derive from having a coelm
Coelom /ˈsēləm/: The body cavity in metazoans, located between the intestinal canal and the body wall.
Annelids are divided into three main groups: Polychaeta, Oligochaeta, and Hirudinea. They possess a segmented body plan characterized by a coelom, which is a fluid-filled body cavity that allows for greater flexibility and movement. Each segment is internally divided by septa and contains structures like muscles and organs, all supported by the coelomic fluid. This body plan facilitates efficient locomotion and the development of specialized systems in these diverse worm-like organisms.