You might have noticed that, sometimes, a shadow in the Background can make the Shape of a face all wrong.  Or a dark shape right near someone's hair can make the hair look untidy.  Shadows, and strange shapes in the background can really play havoc with an image that would otherwise be perfect.  With AI, the problems are worse; some of these programs aren't smart enough to realize that anything is wrong, so the human artist has to be extra vigilant. 

Kay