If you have plates on both the camera and the lens tripod ring (for example, when using one camera with a long lens and another with a normal or wide angle lens), it makes it easier to switch between the two if you didn't have to keep turning the clamp 90 degrees every time you went from one to the other.
it also make it possible to use your camera in portrait orientation when using a tilt head on a monopod when the monopod is connected to the camera.
ed rader

