I looked at a 69 Chevelle that was lowered in the rear. It had the shocks removed and replaced with a coil-over shock system attached to the chassis (the hole in the trunk was bare) and a ladder bar type traction bar was welded to the differential.
What is this type setup called ?
Is it used mainly to lower the car or does it also have to do with providing better traction ?
Thanks.

sounds like a 4-link set-up, they are very adjustable for ride/traction/height.

Could be either 4-link or Ladder-bar setup. I'd never drive a ladder-bar setup on the street.

It has ladder bars and the coil over shocks/springs attached to the frame. Whatever it is it rides and handles nicely on the street. It is lowered in the front too and I have to say it drives and rides better than stock ones I've ridden in.