If photography and movies need to remove the background, why use blue cloth or green cloth as the background? What's the good of this?

General film post-production software has a function, which can dig out what you need from the blue or green background, and then you can easily replace it with the needed background. For example, the last scene of Jackie Chan and Kim Hee Seon flying in the air in the movie Myth was made with this. The specific principle is not very clear, and it may be easy to filter.

Anyway, I have done it, and the software can easily extract images with blue-green background.