Single Note Replacement- Who came up with this term?

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I don't understand how this term Single Note Replacement came about, nor why there is such a large amount of "SNR's" on the market recently.   What's up with that?  If they're so rare why are so many being sold?

What makes a SNR?  If at least a sheet was used why would it be called  a single note replacement (why not a sheet replacement note?) How have brick searchers or "experts" discovered this phenomenon?  Why should we be interested in collecting them?

Thanks in advance for any light that can be shed on this new phenomenon.

This may seem like a strange question ,but how does one, who's main focus is replacement/inserts, really know if one being sold through ebay is in fact genuine? I know that there are various registries for the multiple range inserts to verify their authenticity, however there is no way of knowing if 1 out of a brick of 10,000 is real unless you were the actual finder . Any other members curious or can help me out with this query .

Thanks Mark for your honest input. I'm glad to see that I'm not the only one to question this subject . There certainly does seem, to me too, that there are an awful lot of "new"  AOH insert ranges appearing for sale on ebay .