In olden times (ca. February '88), people used to run their newsreaders on big machines with permanent connections to the net. News transport was dealt with by news servers, and all the newsreaders had to do was to read news. Believe it or not.
Nowadays most people read news and mail at home, and use some sort of modem to connect to the net. To avoid running up huge phone bills, it would be nice to have a way to slurp down all the news and mail, hang up the phone, read for several hours, and then upload any responses you have to make. And then you repeat the procedure.
Of course, you can use news servers for doing this as well. I've used
inn together with
for some years, but doing that's a bore. Moving the news server
functionality up to the newsreader makes sense if you're the only person
reading news on a machine.
Setting up Gnus as an “offline” newsreader is quite simple. In fact, you don't have to configure anything as the agent is now enabled by default (see gnus-agent).
Of course, to use it as such, you have to learn a few new commands.