With a light sensor you could adjust the colour of an LCD display so it exactly simulates the light reflected by paper. I don't understand why nobody did it before. Now you can't patent it though because this comment serves as prior art :)
They're usually used as part of a larger calibration workflow, for example: calibrating your monitor and printer so that what you print with a specific ink/paper combination looks exactly like what you see on the screen. (Or, as much as is technically possible to do so.)
[0] - https://www.xrite.com/categories/calibration-profiling/i1dis...