Twitter friends is an analytics tool which gives you more information on your Twitter social network. What kind of information? Who messages you the most, who you talk to the most, the average number of daily tweets and even your average tweet length. It also allows you to retrieve an archive of all your conversations with a specific user, just by entering his/her username against yours.
It’s useful for a quick way to determine who you interact with daily on Twitter. The focus here is on re-discovering the people that matter, the ones that proactively generate conversation or follow your twitter ramblings (as opposed to uni-directional marketing accounts). The TwitterFriends FAQ explains more:
If you live a liberal following policy (= if another user follows you, you follow back), then the network of your contact does not have any deeper meaning. But there is a smaller, much more meaningful network hidden behind your total web of followers: the relevant net. TwitterFriends helps you to find the users that are meaningful for you and keep in touch with them. Those could be users you are talking to on a regular basis or who are feeding you great links all the time.
From another perspective, a tool like Twitter Friends can also show you how limited your circle of interaction is and may encourage you to actively reach out to people you want to include in your social graph.