Why am I able to cat a directory?

[Stanislav Syekirin]

Hi all,

I accidentally noticed that the following runs without error (even 
though it only outputs some binary data I can't understand) on my 
Dragonfly 6.2 install:

     mkdir a
     cat a

The same happens on NetBSD, so it probably isn't a bug, but I really 
can't imagine the intended use case. Can someone please explain?

Regards
Stanislav