what is this attachment for?

Hi Mike...

It is an edge guide. In the same orientation as is pictured, unscrew the wing nut, place the slot of the brass colored piece onto the wing nut post and screw wing nut back on. The end of the brass colored piece has a small detention that fits over the shaft of the knurl knob allowing for micro adjustments with the wing loosened.

The bearing would ride on the surface of the edge of the board or piece that you wish to trim or profile. The two long rods slide into your router base and would be secured by screws or thumb screws. Slide them in and out for gross adjustments. The bearing end also allows for adjustment so the bearing rides squarely on the edge of the board...

Hope this helps...Nick

Most of never use them and with good reason. As soon as one fence passes an end the bit digs in. If you plan to use it attach a board to fences that is longer so that it is more stable.