This overlay would allow you to specify a margin to allow for framing (e..g 1/4 inch) so that you can properly crop the image and not clip the subject. You may also need to specify a DPI (e.g. 300) for scaling. A full photoshop guides version would be even better!
For me, it's useless, just correct the report length over width, then when printing, you can very well take into account the final dimensions and properly frame your picture in the frame. Print from LR Classic menu.