When i was just starting out as a Dom, i went through a phase where i inhaled every ounce of information i could find on the subject of BDSM and D/s (there's alot out there, and alot of nonsense)
I stumbled across it then
As far as what it does, i think it's more of a fun fantasy than anything that actually matters. I've seen the occasional picture of a sub getting a tattoo or a collar with their number on it/them. It seems fun, not something i personally care too much about.
Its an interesting idea though.
I agree with jgg seems like a bit of fun that some decided to start up.
Though I don't doubt there are some subs who will take it very seriously and some Dom/mes who might think their subs need to be registered there, but there are always people like that.