Shimano started incorporating ball thrust bearings in their road calipers in 1987, with New 105. It was part of their SLR design to reduce system friction and allow lighter return springs. This in turn reduced hand effort and improved modulation. I know the practice continued into the 1990s with dual pivot Super SLR road brakes. Without the thrust bearings, the return spring might not be strong enough to fully release the arms, resulting in slight dragging of the pads on the rims.