One important feature for me is that in a "private" forum you can store links to important threads without having to look them up all the time. Another handy thing about a "private forum" is that the conversation between members is available in one topic. Me and my buddy, who lives in another country, discuss about various things (see my previous posts). The federation forum enables us to keep track of the discussions in different threads.
Well, I hope this helps us keeping our fed.