The WCU-4 with a monitor and receiver (and battery) attached is quite heavy. And the regular hand position on the left side is very far away from the center of gravity. This results in high fatigue on the wrist.
The JOE bracket allows to mount and adjust a monitor to the WCU-4 and has its own handgip which is placed much closer to the center of gravity. The weight is no longer carried by the wirst but by the much stronger biceps.
When the WCU does not need to be carried around the handgrip serves as a docking bracket on a C-stand.
The cam-jam JOE comes with its own quickrelease bracket on the monitor so nothing extra is needed. No noga arms, no cinelock, no spigots.