The rear end being in a "sag" position helps with off the line starts, as the weight of the car shifts back to aid in traction, (in theory). ALSO, for overall aid in stopping, the backend will not rise up on a hard brake, the frontend will go down as well to help the car stop better, (in theory). In reality it seems that 67's were set up different than 68's.
Not a fan of the "sag" look. Not a fan of the "Jacked up reared" look also. I feel that a very slight "rake" makes the car look better standing still and while driving. A sagging car, any year, any brand, any model, just does not look good, (to me).
Probably yet another reason to not want a "concourse" car. (for me).