What are those washers for?
That washer is a part of the "seal" system for the hub.  If you look at the hub there is a lightweight metal cap pressed into the hub over the area of the bearing race, and that washer (which is plastic) fairly snugly fit into the hole in that cap.

This doesn't create a water-tight seal, but it's tight enough to keep out a lot of the dust.

I've never studied closely how the washer is placed there.  I would assume that the cone is made of two pieces that are swaged together during manufacturing, but I can't say for sure.