User blog comment:DaNASCAT/Technical Update: November 8, 2011/@comment-3030984-20111108193647/@comment-1656630-20111108194253

it makes more sense if you think of it as preferred names. For a lot of us, our usernames are awesome. But not everyone loves being called by their usernames, so we're making it possible for them to have more flexibility. We still need to have usernames displayed prominently for lots of reasons, though, so that will continue to be a big part of our interface.