They provide protection against neck injuries in rear-impact collisions by limiting head movement, and have been a requirement of the US federal government since 1967. Rhonda Parker, a Utah Highway Safety Instructor explains that headrests pose a problem if people don't adjust them to fit their size. Only thirty percent of people adjust the headrest to fit their head. Source To be effective, the top of the head restraint should be between the top of your ears and the top of your head. Horizontal placement should be as close to your head as possible, without pushing it forward or causing the height of the head restraint to drop.
Some vehicles have rear seat head restraints which function in the same way.
For more on headrest safety, click here or here for some interesting graphical interpretations of neck injuries. Adjust your mirrors. Most people tend to adjust their mirrors so they can see the sides of their own car while sitting in the driver's seat. That alignment leaves a blind spot for cars that are passing you on either side. To adjust the right mirror, sit in the center of the front seat and move the mirror until you can barely see the right side of the car.