If not, during a emergency panic braking, the whole rear seats with it's contents or passengers can be thrown forward dangerously.
Well, hang on, it's not that dire:
- Yes, if the seats are empty, they could come out in a front end collision, potentially clobbering the front seat passengers. That would be dangerous.
- But if your passengers are belted in, the rear seats aren't going anywhere in a collision.
- Contents (e.g., stuff) lying on the seat is going to go flying in a crash regardless of whether the seat is secured or not.