Seals washer pump to washer reservoir. This is for the smaller 10mm hole. See the various grommets and plugs below, for all different pump configurations.
For both front windshield and rear glass washing systems. In other words, if you have both front and rear washer systems, you will have two identical pumps mounted side by side.
If you also have the headlight washer option, this pump is mounted to the washer reservoir but, it is on the opposite side of the reservoir. See that pump shown below