Srew lock:
To secure the screw for falling out. Mostly used on covers of flashing units or rear light units.

Seal:
When used on the outside of a lamp-frame this washer acts as a seal to protect the inner of the lamp unit gainst damp.

Hint: If the hole is a little bit too narrow to fit the grommet, just use WD40 to get the grommet slippery. This wouldn't harm the grommet and the cables and fitting is much easier.