Models and performers who have been in the field know how to make themselves look good on-camera. They study and learn certain poses, angles and head tilts to emphasize on what they want to show and hide what isn't needed in the shot.
Real people i.e. those who don't get photographed every week or that often (- and no, photo bombers and selfies don't count even if you know how to do 'the duck pose'), can't exactly know, see or imagine how they look behind the camera. Sometimes, a good photographer is the best way to go when creating fabulous photographs.
Posing is key; not just any pose, but a pose that shows an aesthetically good design. Photographers aren't just there to press the shutter, they exist to help their subjects feel good to look good.
Posing children over 4, I find, is not that difficult as they take directions quite easily. Showing or acting it out makes it easier for then to understand. The thing about children though, everything needs to be quick or you lose the shot.
Photographing teenagers and above might be either both difficult or fairly easy. The key is to again make your subject comfortable by talking to them and making the experience fun. Once you feel that the connection is there, "Boom!" Take the shot.
Posing I believe is about connecting to your subject and building that relationship. Every moment counts and never miss it.