Why eight washers in windage tray install kit P3690939?

The washers can be use to avoid twisting or brinnelling the tray by putting them under the bolt heads. I personally would not be to worried about losing 1 thread of engagement but others may opine differently. The stress on the threads in the block at standard torques is well below the pull-out stress; the threads still hold up with a stress increase of 50-70% when lubed ARP studs are installed.