It provides light through a sheet of paper or film so you can trace. It allows you to view items like an x-ray film, photo slide, photo negatives, etc. I bought it so I can draw my pictures, and then trace the finished work on card stock to make greeting cards without the need to erase sketch pencil marks.
https://www.amazon.com/Ultra-thin-Brightness-Animation-Sketching-Stencilling/dp/B0771J6JY4

