https://en.wikipedia.org/wiki/Usenet
https://en.wikipedia.org/wiki/Nntp
As for "why pay separate provider" -- that would be because circa 2000 most ISP's dropped offering Usenet access for their customers as part of the deal, which created a market for the paid providers.
But, you don't /have/ to pay:
https://greycoder.com/best-free-usenet-servers/