People use white screen for:
For video calls! I don't have a easy ring light, but do have two screens. So I can use one of the screens to light my face!
I needed to have something bright under a paper, so i could redraw a image easier.
Lighting up the back of a lightbox for product photography
Check your monitor and find dead pixels.
Don't want to be distrupted? Want to take a brake? Use a white screen to blank your screen.
To get a source of light and be able to read books when it’s dark in the room and the light switch is too far away.
As a bright light source for doing makeup or taking pictures
Silly, but I use it for monitor cleaning. It makes things on the screen obvious. :) I also use it to validate plain colour reproduction for different monitors, but that only comes up rarely and I'm not looking for professional colour grading.
Consistent backdrop to clean my monitor on, and also to check for dead/stuck pixels. Also sometimes for lighting my face when I'm using a webcam.
I was catching little flies as they were attracted to white screen on my display
I'm working on a animated sequence, and i'm doing it the traditional way (by hand), so i need a white surface with light that allows me to put one piece of paper over another and be able to see both, so since i don't have a professional screen for this, i'm using my computer and putting the white screen on it so i don't touch anything by mistake
While studying, I use 2 screens, one with a timer and one to search whatever I need on the internet. When I am not searching, I put the white screen so I don't get distracted with anything on the computer
Useful when adjusting settings in my secondary monitor (since it's quite old it doesn't just set stuff how it should be by default). Settings like vertical/horizontal position and vertical/horizontal scale. Also useful when searching for dead pixels.
If I'm drawing on paper, I can put paper on top of my display drawing tablet and see what I'm drawing at night