I would give credit where credit is due but I'm not sure who made it over at UM HQ. Speak up if you're around :)

This is a simple tool to make it easier to set the correct distance between the top of the base plate and the bottom of the heater plate. This is important in order for the automatic bed levelling to function properly.

Normally you should't need to use this but if your bed has become badly misaligned this is a quick way to get back to a good starting point before performing a manual bed levelling procedure.