Why is fastening the safety seat belt (Sicherheitsgurt) vital when driving a tractor equipped with a safety cab or roll bar (ROPS)?

1. It stops the driver sliding off the seat when working across slopes
2. It reduces the load carried by the roll bar during an overturn
3. It is required only where the tractor has a folding roll bar
4. It keeps the driver inside the protected survival space of the ROPS during an overturn, instead of being thrown out and crushed