People use green screen for:
Photoshop myself in my room so when i take it down and im still in my room people are like oh wow for a few seconds which makes me happy because am in a way the wow factor of those few seconds of their lives.
In a class room situation, while pausing computer work and listening to others. Green, because it is the most relaxing of the colours. It is just a silent, motionless green colour, the colour of trees and grass. Thanks to it, the screen emits some green, relaxing light during computer work pauses.
For adding a green glow during meeting.