User blog comment:Wally Valdez/Who Could Seth Be?/@comment-6677347-20140810000004/@comment-6331988-20140810000411

It's to avoid spoiling the identity of the characters. If an actor saw the casting call, he'd know the kind of character he was auditioning for, but not the character's actual name. Then, if he gets the role, they will tell him who he is actually playing.