Agents must depend on the kindness of the casting directors to see their actors. Casting directors get several hundred invitations a year to plays, grand openings and, of course, to showcases that are held all over Hollywood. When I growing up, we wanted to be just like John Wayne or Elvis. All the girls wanted to be like Marlyn Monroe or Elizabeth Taylor. If you are an actor, you know all too well how important a casting director can be to your career.
Actors spend most of their lives trying to get in to see casting directors.
It's not that casting directors are such important people and they are no better than anyone else. And it's a fact that they don't always make the final decision of who gets the job. The casting director first sees many actors for each role he is casting. He then makes a choice of which actors he will bring in to meet the producer and director. This is called a callback.