Green is the channel your eyes read first
Human vision is most sensitive in the green part of the spectrum, which is why camera sensors carry twice as many green photosites as red or blue, and why luminance is calculated mostly from green. For screen testing that has a practical consequence: uniformity problems, banding and dirty screen effect are more visible on a green field than on a red or blue one. If a panel looks even on green, it is probably even.
It also isolates the green subpixel. A pixel with a dead green element vanishes on this screen while looking almost normal on white.
Using a spare screen as a chroma backdrop
A tablet or a second monitor showing full screen green can stand in for a chroma key backdrop when the subject is small: a hand, a product, a puppet, a single face on a tight crop. It will never replace a properly lit cloth for a full body shot, because a screen is an emissive surface and spills its own light onto whatever is in front of it.
If you try it, turn the brightness down rather than up. The usual failure is green spill along the edges of the subject, and spill scales with how much light the backdrop is throwing. Keep the subject as far from the screen as the frame allows, and light the subject separately.
The other green screen
If you arrived here because a video call, a game or a stream is showing a green picture and you did not ask it to, that is a different problem and this page will not fix it. A green cast over video usually means a hardware acceleration or codec fault, and the first thing to try is disabling hardware acceleration in the browser or player. A green flicker across the whole desktop points at the graphics driver or the cable. Neither is a panel fault, and both will look perfectly fine on this test page.