What is the significance of the fact that the bones of the pelvis are connected motionlessly?

The fixed connection of bones performs a protective function, protecting the internal organs (genitals, in this case) from mechanical damage, since such a connection is the most durable.
The pelvic bones are part of the musculoskeletal system. And directly perform the main function. The pelvic bones are completely fused to a certain age of a person, a stage of development. Thanks to special hip joints, the bones of the free lower limb are connected to the pelvis. I believe that the bones of the pelvis are connected motionlessly in order to perform the most stable movement.