Passive Preamp Build; ? on RCA Jack Grounding

I'm starting a build on a small passive preamp. The RCA jacks that I bought have two small insulating washers. One has a shoulder on it. It makes me think that the washer with the shoulder is designed to center the jack in the hole and prevent it from grounding out through the rear panel.

This didn't quite make sense to me since I will be attaching the outer shell of the jack to ground anyway.

Is there a reason to use the washers and to prevent the jack from directly contacting the panel?

Thanks,

Chris