This is obviously a specimen but in real life you do this when somebody broke something in the joint.
You would think, why saw another bone and 'break' it like the first one?
Why not just approach the broken parts (like the radius or the humerus in the joint) from the other side?
As you can see here, there are nearly no important soft parts like veins, nerves or arteries around the area of the joint. The only important one is easy to secure (it is the N. Ulnaris which is pulled down here). But on the inside of the ellbow joint you have all that. So it is easier for a surgeon to saw trough a bone than operate through a net of very important soft parts.