I believe the washers are ECE regulation for low beam lights, if the light output is higher than 2000 lumens.
It is not specific to xenons, but most xenons have light output more than 2000 lumens and halogens will be less.

Washers go under the headlight, between headlight and bumper.
See the little flap under headlight in this picture.
https://3a663eb0fef48c6d2d60-a88f8eb...1360_x600.webp