People use black screen for:
Quickly make second monitor dark without turning it off when playin some atmospheric game or watching some movie that turned out to be worth of doing so
Trying to save electricity, reduce carbon emission.
To put on my 4 monitors when I go to sleep, instead of putting them to sleep.
Easier to see spots when I'm wiping off my screen
Background for a game. Screen is too big, 3440x1440. Don't like to play it that big, so i do 2560x1440. Need black bezels at both sides.
I have an old plasma TV. I don't have computer speakers so when I want to play music loud, I turn on my TV and hook it up via HDMI. Plasma's are highly susceptible to burn-in so rather than leaving Windows Media Player or whatever on the screen, I use black screen to help prevent burn in. I've used various websites. But they don't seem to stay online for very long.
To not turn off my screen and concentrate while studying. Well my screen is very bright and it doesnt help with concentration, a black screen fixes that.
Because I need to relax my eyes and have just a black screen even when I play music on my pc or want to switch quickly between all black and something else. The most valuable is the quality of it, of the color and the rest and, of course, for it to be easy to switch in and out of fullscreen mode.