What is the purpose of this grounding screw in a plastic electrical box?
The bare ground wire in the electrical box is supposed to connect to the green screw on that short metal bar. The body of the light fixture is meant to be grounded through the mounting screw that goes into that metal bar. Do be aware that this technique of grounding the light fixture would have been the technique used in older installations. These days a light fixture will have a separate green or bare grounding wire that can be connected directly to the bare copper wire in the electrical box. 

You may want to inspect that old light fixture and see if there is an existing screw or mounting hole to which you can add a separate grounding wire. You would then connect this added wire to the bare ground wire in the electrical box.