only thing I could think of, is that the leaf springs spread the load/weight of the car out over 2 points, and adds load to the rear frame rails, vs having it all loaded on one point with the rear frame rails/trunk floor, etc. unloaded and just hanging free.

When I did mine, it had rear suspension installed and car sitting on it's own weight (jack stands under rear end) I had to push in a little on the tail panel because I had to replace the trunk opening gutter rail at the same time, when I cut it out, the tail panel "sprung" outward, even though the replacement piece was a perfect match from AMD