The entire point in creating a list on Twitter is two-fold - they are for you to create for your own benefit and to aid in other people discovering other tweeters from the lists that they find interesting. For example, I find I tend to follow a lot of people who do web development, and I also follow colleagues and quite often do reciprocal follows too. This does mean that my main Twitter stream can get pretty crowded, and whilst I'm asleep I can usually guarantee there will be more tweets from people I follow than what I'm allowed to collect via the API. What this means is that I could miss potentially interesting tweets from the people I'm the most interested in reading about.
In TweetDeck on the desktop it's great that there has been the ability to create groups for a while which meant I could create a group solely for updates from colleagues - now I can replicate this using Twitter Lists so I can see what they're tweeting about.
I think it's like having a filter really more than it is about having bragging rights to how many lists you appear on. A very nice and potentially useful feature from the Twitter team.
