Yes, they do what they are supposed to do (increase extractor tension). BUT they are only to be used with a standard-weight extractor spring (that is, if you are running an extra-power extractor spring, the O-ring is not needed and may over-tension the extractor).
What brand is your BCG? Most companies don't include O-rings as standard equipment.
All of my Bolt carriers came with that o-ring on the extractor spring. I think it is becoming a lot more common. Multiple places of origin on the bolts, none of them were high end.