Sometimes on set, there just isn't enough space for the director or DOP to get near to the camera, especially if you are using a crane or Jib Arm, or the camera is on a gimbal.
With camera gimbals, it also be comes very difficult to run a cable to a monitor as it poses a trip harzard.
For these situations we have built a number wireless monito kits, with HDMI and SDI versions to woth with your camera set-up.