Seat belts also position you and HOLD you behind the steering wheel and controls, and that in itself is valuable. When in HS, coming home from a HS basketball game one Friday night, I was turning left onto a secondary road (from a more major highway), and about halfway thru the turn I was hit by a car attempting to pass me and several other cars. My car was probably traveling at 30-35 mph when I was hit. The collision knocked me into the back seat floorboard (Yes, over the seat back)! My car (a '57 model) was still moving down the road without a driver! Fortunately a friend was riding with me and was able to reach over and at least steer the car down the road until we stopped without going into the ditch! I also remember losing control of that same car going around a 90 deg turn on wet pavement and going into a series of the rear going way out of control, (I was holding onto the steering wheel but my butt was trying to slide across the seat each time I'd steer into the skid and it would go the opposite way!).. So seatbelts DO HELP and could save your life by keeping you in the right position behind the controls.
PS. I believe the 1956 Chevrolet models were the FIRST year that seat belts were even OPTIONAL in a Chevrolet (although Cadillacs may have offered them a bit earlier), and I think the most common type of aftermarket seat belts 'mimic' the look/function of that first 1956 seat belt!