Why green screen?
Green screen is used for anything that would require the background to be changed.
The technique has been used in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture, and video game industries.
This technique is also referred to as colour keying or chroma key.
Chroma keying can be done with backgrounds of any colour that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colours.