The Internet is a network of networks. Nowadays, everyone tends to have always-on connections to the entire thing, but back in the day, many of the networks spent a lot of time disconnected from each other. Usenet was designed to mostly transparently handle this by the local network having an aggregating server that would collect all the local activity and share it with other Usenet servers when it could reach them.

Remember that even the local activity was people connecting with teletype terminals and dialing up over modems from remote systems. Long distance trunking fees were a big deal, and Internet routing had to deal with the possibility that there was currently no route to the destination address.