As I mentioned, they were used only on hardtops and convertibles. Purpose was to assist in reducing door sag and misalignment due to flexing of the body. I also question whether they really did as intended to any great extent. GM must have thought they did.
Several GM cars used them. On a Corvette, if you leave them off, the top edge of the door will bang against the top of the quarter jamb and chip up the paint unless you have a large door gap. I would presume that the same is true on most convertibles as well as hardtops of unibody design. Sedans have extra rigidity from the B pillar. On my 66, the screw holes were wallowed out, so I bonded a piece of 16 ga metal behind the holes in my doors with panel bonding adhesive. The originals were only sheetmetal screws screwed into the jamb.
I am going reach up in the quarters and bond a doubler in that area as well. Corvettes have a thick plate that was threaded to accept machine screws, and they never failed. I am going to look an my buddy's 67 Camaro convertible this weekend to see how they treated the attachment on that body.
If you adjust them properly, they really do make a difference in the rattling of the door. You just keep adding shims until you see the door gap increase a little when the door is closed, and then it is adjusted right. If you look at them, you will see that they are wedge shaped and the plastic wedge slides easier against the metal wedge as the door closes. If they ain't tight against each other with the door closed, then they are not properly adjusted.