What Is It? Do I Need It??

Ok, I'm not the best photographer, but can anyone tell me what this is used for so I can figure out if I need it ?? It's flat on top and comes to a point on one end. It slides side to side by turning the knob on the other end. I'm not sure what all else it may do. I thought at first it may accompany a rotating vise I bought from the same guy, but it is from a different...

What Is It? Do I Need It??