I use it for when I want to speak to someone privately (err, that sounds a bit flippant!).
There are often times when I want to say something that I don't want the group to see, usually because it's an off-the-cuff thing that I don't want people to get offended by, or because I want to do something that I'm not sure is OK, and so I want to have a quiet word with the relevant person(s).