You can use greenscreen wherever you like, although it is often easiest to use it in a studio. The entire point of greenscreen is that it enables you to photograph everything in a studio or controlled environment, then composite the subject onto any other background of your choice.
If you've already photographed a subject on location or without a greenscreen, generally that background should already be the correct one.
Greenscreen is specifically for putting your subject into places that your resources won't allow (whether that's due to money, travel, equipment, time etc). PhotoKey is designed to make this process as simple as possible.
You can find out more about it here:http://en.wikipedia.org/wiki/Bluescreen