Usenet is many things, like email is smtp, or websites use http, usenet is nntp. I can explain mostly from a piracy aspect.
Some providers run servers which host archives that go back years, some host "binaries" (which can be stuff like software or movies) while others like google will only do text based content. The reason you need a provider is because storing decades worth of binary files takes a lot of space and that costs money.
The reason you want long retention is so you can find all the episodes of that show you want to watch that aired 10 years ago, or a movie that was last uploaded in 2015
Then there are usenet indexers like drunkenslug and nzb.su, they are similar to torrent trackers in that they have files with the information needed to locate a given binary file, most content is split into segments of 10s or 100s of mb