As describe on upper posts by Quante and Scottrik, the spacer is used to apply an axial load to increase or release some pressure with the nut on the differential, to tighten or loosen it.

I did use plastic because first that was I have easier access, it's easier to machine and, in case of something move in the assembly, that would avoid some sparks made by a metal-to-metal contact. Last point, Nylon get some interesting friction properties.

If you click the pictures, you'll get some details. The lip helps to make some clearance with the bearing. In fact, if the lip wasn't there, the spacer will took both bearing's rings. As the internal diameter of the spacer is slightly larger than the axle diameter, the spacer is fairly well positioned and the lip on the outer diameter give space to a avoid contact with the inner bearing's ring.

Hope that well explained!