The washer is so small, compared to the size of the float bowl, it would be hard for it to keep much fuel from sloshing. Perhaps it's there to protect the idle jet from getting splashed when cutting the throttle for a quick stop, and killing the engine. The idle jet is right below the washer.
Still not sure where the author of the PHBG tuning site on Moped Wiki got that the washer should be removed. He said "Remove it because it often blocks fuel to the main jet and causes extreme lean conditions. "