For the moment, it's used for nothing
For the moment, you can set it only for a person, but soon you'll be able to set it for a movie too
When it'll be working, hopefully in a few months, it'll survey each database update. For instance, you're a big fan of a person, and have activated the video surveillance for this person. If the new update has a new movie with this person, it'll trigger a notification message. Second example, you've activated the video surveillance for a movie. You'll be informed if a new video of this movie has appeared (might be a new edition, or the very first video if the movie was just released on theater). Of course, you'll be able to specify the type of videos you want to survey (for instance, get alerts for DVDs, but not for UMDs).
You might think it's strange... It is, I admit