Those pieces compose the adjustable bearing spacer. They allow the bearings to be fully snugged up, but yet have minimal preload, thus drag. In other words, less friction. You can actually feel the difference pushing the car back and forth in the shop.
As the bearings are still side loaded in the corners, the benefits likely are mostly in the straights. I suspect they add some minimal stiffening to the spindles as well. The bottom line is they free up some horsepower which should help in the acceleration. They definitely reduce the bearing temperatures. Taking into account the small bearings we have on the early Camaros, it all helps.