This is a switch mount for the power supply for the Printrbot, its to make it easier to switch your bot on and off. It mounts on either side of the printrbot using the same hole that you put in a screw to attach the top of the base to the side of the base. (You just need a bit of a longer screw)
EDIT: Thinking of it now, you could also actually attach it on the side of the bridge. Don't know if that's handy, but the size of it fits perfectly in alignment (for good looks)
There are two missing holes in the model, I was too lazy to calculate the exact location of the holes and instead drilled them out once printed. You can do it like I did or calculate where those two holes are supposed to be located and create a derivative of the model.