From my understanding, if the hot wire (black) were to come in contact with a piece of metal on the fan it would build up electrical charge and or allow for current to pass through the piece of the fan. Then if you were on a ladder fixing the fan and touched that piece of metal you would become a path for electricity to pass through to a lower potential.
The way you bond is codified too. The source feed is wire-nutted with a pigtail connected to the box, and to the ground of the device in parallel. If the bonding was daisy chained it could be interrupted when servicing or changing.
