What are Grey Balls used for? I know that a Grey Card is used for the white balance of the camera, but whate are the Grey Balls used for?

On big Productions, there is usually a guy (what is his job?) that holds up a grey ball and a "mirror"-colored/reflective ball.
What is this used for? Is this for digital effects added in post and to see the light and the direction of light?
What is the purpose of them?

Also, sometimes, there is the use of a reference card (a paper with many little colored squares on there).
What is that used for?

Is this to adjust the color of the digitally added material?

This can't be added to adjust the images in post, because they should be the same (because of white balance), is it? Does dynamic range matter with these kind of things?

What are these balls called and what for are they used?
What Role/Job does the person holding them have? Does he has other duties or is he/her just holding them?
Yes, they are used for adding visual effects in post. The mirrored ball is for creating an environment map or reflection map. These maps provide the same lighting from the real scene to the virtual scene where graphics are created to composite into the existing scene.

I believe the grey ball is used for recording more diffuse lighting from all angles, though I'm not positive.

The cards with either black and white or colored square patterns on them are for motion tracking, image stabilization, and orientation. They're often used to replace computer or mobile phone screens, for example.