At the time last fall, one of the main things was that it provided audible and Notification bar notifications, something K9 didn't. It looked a bit more sophisticated, too. Ultimately it's just another email app, with things like auto account set up (which K9 I think has now?).
EDIT: Actually, one nice feature is the notification profiles. For example, you can set it up to be silent overnight so it doesn't wake you up in the middle of the night when an email arrives.