What is the legal purpose of the headrest (apoyacabezas) in an automotive seat?

1. To provide a comfortable surface for the driver to rest their head while sleeping during long journeys
2. To prevent severe cervical hyperextension (whiplash / latigazo cervical) in rear-end collisions
3. To serve as a mounting anchor for aftermarket rear passenger entertainment screens
4. To increase the structural height of the seat frame for aerodynamic efficiency