Hey feenixfire,
that are one of the most important parts of your CNC machine. They allow you to adjust your machine for coplanarity, which is extremely important on gantry-style CNC machines, which have two Y axes at a greater distance one from the other. If your machine is not adjusted for coplanarity, it will only produce twisted workpieces.
That is probably also the reason why Onefinity now includes them with the QCW frame (before, you had to order them separately here)
Welcome to the forum!