Silly question about wheel center caps

I had one of my center caps start to delaminate, so I decided to get a new one. When I tried to order it, I realized that there is a new design. They are a darker yellow, and grey plastic instead of black. Due to this, I had to buy 4 new ones.
Now for the question:
The old caps had a rubber O-ring on the base of it, and the new ones do not. When I took my wheels off to replace my tires, I tried putting the center caps on, with, and without, the O-ring. They seem to go on fine while using the O-rings, but they have a more secure sounding "click" without them when they are snapped in place.
I have no idea why they are there in the first place, so it's hard for me to determine if they are needed.
Am I supposed to re-use the O-rings, or put the caps on without them?
I told you it was a silly question! LOL

 

Thanks for thr replies! I will use the O-rings when I replace them.

I like the new design, they are a little darker yellow, and brighter colors. They seem way more durable because the edge sort of wraps around so they also look sleeker. The old ones seem more fragile, and the center disc have a tendancy to come loose.

 

Actually for some reason, mine is the opposite, the shield looked closer to the old lighter colored caps. My shields are the factory shields, so I don't know if that makes a difference.

 

I did the same thing, but now that I think about it, I feel bad because I could have given the other 3 that were good to other Fchat members that needed them to replace a damaged cap on their car. I guess if I replaced the tires back then, that's how I would have done it, but I got lazy and figured they were useless anyway.