I could be wrong but it seems to convert OpenGL screen space positions into application screen positions. When points are rendered they are transformed into the range of (-1 -> 1) for x and y positions. If your screen position is (0;0) then your opengl position is (-1;-1) and if your max resolution is (800;600) then the opengl position will be (1;1) at that point. Basically what the function is doing is taking the opengl co-ordinate in the shader and converts it into a screen space co-ordinate as the function is named.
Could be to reduce artifacts caused by the shader, you can test it out by comparing (-1;1) with (-1.3;1.3) clamping to identify the need to clamp over the border of the screen.