Thanks all so far. I hadn't considered the objectification angle because, in general, me and mine don't deal in kink all that much but yes, that makes sense if that was someone's fantasy.
Inosfar as it being a symbol like a collar, ring, etc. I'd be fine with that, but I'm kind of struggling as to how that symbolism got loaded onto it for the people involved. It just seems so... impersonal... at least to me anyway. That's why I posted here. I mean, seriously. For a wedding ring, there is a specific object. There is an elaborate ceremony attached to the giving of that object. The whole thing symbolizes a commonly understood thing (marriage). So it's easy to see how wedding rings get loaded with emotional signficance (mine has only come off once, during surgery, we timed how long it was off and it was put on again before I even reached consciousness... and I STILL hated it being off. Hers has never come off. For either of us, the thought of taking off our wedding rings is emotionally traumatic.) I could say the same thing about collars.
In the end, there may be no answer to that... It could be that for whatever reasons, two people decide that a registry number has emotional significance and so presto, it does.