Dutch doors add flexibility -- for fresh air and easy communication while maintaining privacy and security.
Dutch doors have two operable panels: the lower one can remain fixed in the door frame while the upper one swings open. It's an old idea -- also used for horse stalls in stables -- that is making a comeback, as seen at the 2014 Home Builder Show.
The example shown above is from Jeld-Wen, a major door and window manufacturer: used as an exterior door it allows air flow and visual communication while retaining a measure of security.
According to Jeld-Wen, Dutch doors are also often used on the interior. Here's an image from the Jeld-Wen website showing some of the options available.