Support is just common sense - when you have a long, heavy board, and the machine needs to measure the entire length, think about what happens when all but 3.5" of that board is sticking out of one end or the other - waaay too much weight for the machine to handle. The end comes off the tracking roller, compresses the back or front roller too much, etc. So, you put a couple of rollers about three feet or so from each end to keep the board level through the machine.