You can see what's happening while you're away. Ideally it would only turn on when the normal occupants aren't home. I wanted one when we were out of town and our security system triggered a movement alarm inside. Outside cameras didn't catch anything and a cop came and looked around but didn't see anything. I'm guessing the motion sensor picked up on the curtains moving or something.
Otherwise, it's pretty handy for proving that you told someone something 3 weeks ago to win an argument. /s