Do you mean the side part of the base, see snap shot below
If so it's made so you can run the router along a wall to trim the formica top or the plywood so it will fit flush to the wall, many walls are not true.
You could call it a copy cat device
,the top would be on sticks to hold it up off the base and the router would be held to the wall and pulled along the top so the formica is trimed true to the wall.
Just a NOTE ****** many people don't use it because they put on a back stop on the back side on the top, to fix the error or to fit the top ,it can be tricky in the corners when you have a full size counter top but with block and a clamp on the top side of the top it can be done easy when the top is off the sub base.
Other ways to use it ▼
I should note that it's nice tool to have when you are buliding built in cabinets most cabinets have a face frame on them and the wall ends have a over hang (over size lip,1/4" to 1/2" ) so you can get it to fit the wall side true and clean, most will use a jig saw/hand plane/hand saw.small trim power saw to remove this little part but you know how hard it can be to remove a little edge from the side of stock clean, the trim router with the jig on the side can do this job easy and clean .
Just screw it to the wall for a temp hold job, set the device on the side of the trim router with the adjustment knobs,lock them down then,
Start at the low point and move to the high point and you will have a nice edge to fit the wall,most trim routers run at a very high speed to give you a clean job with the right bit in the router.
Some will let the adjustment float abit and use two hands when you have a bump in the drywall, it will copy the wall to a tee on the side of the cabinet edge.
Molding is made to hide the errors in walls and cutting of the stock, quick fix it's like Bondo on cars,hide the errors and to fill/cover the gap .