Grommets are either fillers or sleeves for wirs to prevent them being cut or worn away on cases or boards. They can also be used as makeshift springs for screw or nut and bolt fixtures. Washers are simply meant to act as seals or spacers to pevent damage or the escape of water or air. A key diffference is grommets have a 'slot' or groove cut into the depth of the ring which then slot into the holes in enclosures say, to keep cables and sharp edges seperate.