It says "Frequently bought together" with a mounting plate - but already have one so why? just to sell it or is it go off so fast so you need an extra
From what I can tell, the suggested mounting plate is a quick release. That would be a matter of preference and you wouldn't need another if you are satisfied with your mounting plate.
In case you have more than one camera and don't want to have to keep moving the plate from camera to camera.