Its only purpose is to prevent the spring from penetrating the rubber gasket on the filter. The OEM washer is unusually thin, but a thicker washer will serve. By using a thicker washer, it will increase the pressure on the filter by a very small amount. Other than that, there is nothing remarkably special about it (use a replacement with the correct diameter, however). Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII