From the Velo-Orange website:
"Essential to the proper mounting of metal fenders, they are also useful for racks and other accessories.
Use one or two of the washers between the fender and the fork crown, the seat stay bridge, the chain stay bridge, and between the fender and racks mounted to the fender. They are intended to dampen vibration, lessen the likelihood of stress cracks, and reduce noise from pebbles and twigs bouncing into the fenders.
The price is for a pack of six, colors may vary. OD is 12mm; ID is 5mm."