For example, you can get some idea as to what your friends are up to all day, what sorts of tools they use, etc.
Imagine for example something like http://twittervision.com/ that one can monitor with respect to certain apps.
netstat 15/tcp
Now I feel like an old guy - I remember when you would occasionally find people leaving that open.
1) firefox 2) top 3) a hundred processes owned by root. boring.
In fact, it would be fun if the service were to make predictions about what you will do next. People could then watch live to see if those predictions come true.