What is the point of fusion of the three bones forming a coxal bone?

The fully developed, adult coxal bone (or innominate bone or hip bone) is made up of three parts: The ischium, the ilium and the pubis. These three bones meet at the triradiate cartilage in an immature individual and eventually fuse as the individual reaches skeletal maturity (15-16 years old). The triradiate cartilage can be found at the base of the acetabulum.