The pectoral girdle helps our movements to be more fluid. Our shoulder blades on our backs are connected to the clavicles out front, so the whole thing moves as one.
Everything in the human body works together, so the load is spread out. The pelvic girdle helps distribute our upper body weight to the legs, so no one section has to take on too much.
I guess the body is like one big warehouse with workers divided into departments. Each section does its own job, but in doing this, it helps out the whole warehouse.