Web-based discussion forums are getting more and more popular. On many subjects, the web-based forums have become the most important forums, eclipsing the importance of mailing lists and news groups. The reason is easy to understand—they are friendly to new users; you just point and click, and there's the discussion. With mailing lists, you have to go through a cumbersome subscription procedure, and most people don't even know what a news group is.
The problem with this scenario is that web browsers are not very good at being newsreaders. They do not keep track of what articles you've read; they do not allow you to score on subjects you're interested in; they do not allow off-line browsing; they require you to click around and drive you mad in the end.
So—if web browsers suck at reading discussion forums, why not use Gnus to do it instead?
Gnus has been getting a bit of a collection of back ends for providing interfaces to these sources.
The main caveat with all these web sources is that they probably won't work for a very long time. Gleaning information from the HTML data is guesswork at best, and when the layout is altered, the Gnus back end will fail. If you have reasonably new versions of these back ends, though, you should be ok.
One thing all these Web methods have in common is that the Web sources are often down, unavailable or just plain too slow to be fun. In those cases, it makes a lot of sense to let the Gnus Agent (see Gnus Unplugged) handle downloading articles, and then you can read them at leisure from your local disk. No more World Wide Wait for you.