It don't see how that would improve image quality.
Try this, get a sheet of clear film and put a mark on it, draw on it with a sharpie, then hold it against the lens, you wont see the mark, now move it closer to the enlarging easel, the closer it gets to the easel the mark will start coming into focus, I believe that this is the reason for the advice and I agree with the advice. I would never put anything between the lens and the paper but that's just me. Anything between the lens and the paper has the potential to degrade image quality.
It is said that we remember the important things, if true, why photograph? I forget, so I photograph.