When a pro video team is recording video, what is hapening on the Mac laptop they have plugged into the camera?

A couple reasons. To avoid needing to swap the card. And another is to see the video on a larger screen to spot any problems. It is best to notice any weird things such as a bird flying into the window as quickly as possible so you can start over.

Sometimes folks will control some camera functions from the laptop, but probably not in an interview situation (which has pretty stable conditions such as lighting).

Factoid:

Jerry Lewis—you know, the comic once teamed with Dean Martin—is credited with inventing the TV video monitor as a way to see the camera setup as it would look on a big screen. Piping it through a laptop is just a modern version of that.

There has to be more to it. Why not just use an external monitor? Why was Finalcut Pro involved?

Having worked in a TV studio, recording to a Mac rather than an SD or P2 card has a few benefits. If the conditions for the shoot are stable, it’s nice to be able to see details on a larger display than the camera’s LCD, as to adjust minute variables to create the best results. Also, being able to record directly into Final Cut, or other editing software, eliminates transfer time.