NNTP descends from UUCP, the Unix to Unix Copy Protocol. Used to be that some businesses and universities configured their Unix machines to call other machines on their modems overnight (the rates were cheaper) and synchronize files. Think of it like a very slow, ancient version of rsync. You would post on your school's Usenet server, which would then sync with one or more servers overnight. And eventually (hopefully) your message would spread across the whole network. This could take days.

Some of the big timesharing services (think Telenet and Tymnet, eventually even AOL) out there ran their own servers, which a lot of smaller services dialed into.

How do you join the mesh? You met someone at Usenix or a similar conference and said "I'm sitting on a whopping 800mb of storage and a T1 line at my university. I'm tired of waiting a week to get the new comp.lang.c hotness. Think I could hook up to your machine for Usenet?" and they'd be like "Cool! Yeah, here's the dialup number. Try to call after 11pm so my boss doesn't know about it."

NNTP took this concept of syncing files and made it Internet native and specific to Usenet. But the architecture remained largely the same.