These are basically third hands. When trying to clamp stock to the Mortise Rail it can be difficult (or comical) trying to get everything lined up and clamped correctly. By resting the workpiece on these lifts it frees your hands and allows the stock to be clamped up much more easily.

Note that there is a standard and a reduced height version of these lifts shown. The lower height version may be required if the beam clamp were used or for other reasons. These are highly effective accessories and would be made in pairs.