There is more to lighting a convincing composite than simply matching the direction and color of the lights on the background. It is not immediately obvious, but for practical purposes, a person on a blue stage is (from a lighting standpoint) standing on and/or in front of black velvet. Since the matting process drops out the blue backing and the blue kick from the edges of the FG object, the object may as well have been in a black stage. This blackness causes no problem if the background scene is a night scene that is essentially dark.
However, if the background is to be a light day scene, then if the person had really been in that day environment, that environment would have provided back and edge light well as reflected light to light up the hair and to provide the normal edge brightness along arms, sides of the face, etc. The cinematographer must back- and side-light the subject to provide about the same amount and direction of lighting the environment would have provided. If this is not done, edges of arms and legs and faces go dark and the scene looks like a cutout.
Inappropriate lighting will compromise a shot the instant it comes on the screen, while faulty compositing technique may be noticeable only to experts.
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