The collarbone is one of the few bones which will fail to heal if not allowed to set properly.
Treatment depends on the type of fracture.
With a greenstick or hairline, one can forgo a figure-8 brace as long as the rider is careful not to stress the shoulder. However, any simple or compound fracture needs to be immobilized. Motion in an unset fracture can cause excess calcification with a noticeable bump or an abnormal set angle. These are actually quite common as the brace is uncomfortable and it's even more uncomfortable to sleep on one side only. This can be remedied with a little tape and a couple of thumbtacks.