The paper feeder drags the paper across that skinny piece of circled glass. The scan light stays directly beneath it.
So, if for instance you were to put a dab of whiteout on that glass, when you made a copy there would be a perfect solid line straight across the output. Because the whiteout was always there between the light and the paper.
As opposed to a copy on the glass, where the paper is stationary and the light moves.