In this kind of casting, the producer casts either without regard to type, or sometimes against type. One advantage of non-traditional casting is that it makes us see the play in a different way. As an audience we have a tendency to come up with explanations for strange things which are in our world. Most of us wouldn't really have a great deal of difficulty accepting an African American as Hamlet if he were good enough in the role. After all the role has been played by a number of women, including such major stars of past eras as Sarah Bernhardt and Judith Anderson.
Directors have much more difficulty with a choice like that than audiences do.
On the other hand, an African American Blanche Dubois in Streetcar Named Desire makes us readjust our ideas about the play. Streetcar takes place in New Orleans at a time of strict racial segregation. Blanche implies that she was a Southern Belle and that her family lived on a plantation. It would make us look at the play in a different and probably a fascinating way. Character is essential to theatre.