They are used when it is not possible to install spacers from the gearbox side. For example this is MK5 2.5 with automatic gearbox, Jetta mk6 with automatic 6 speed gearbox. OEM driveshaft part number: 1K0407271BP. Thickness 12 mm.
After spacers are installed, the length of the spline connection with the bearing hub will be reduced by 12 mm.