What does James Baldwin achieve by beginning "Sonny's Blues" as he does?

James Baldwin's "Sonny's Blues" begins in a way we call in medias res. This means he begins in the middle of his story. From there, the story goes back and forth in time.  Beginning this way draws the reader in well as we wonder who this person is, what he has read about in the paper, and what will happen next. We want to peer over his shoulder to see the article.  We can visualize this scene.  Even for someone who has never been on a subway, this is drawn fairly vividly, the man's pale face appearing as a reflection in the window as the subway hurtles through its underground paths.  This beginning passage also foreshadows and sets a certain tone on the part of the narrator. He is not identified in the beginning, nor is he ever identified, except as Sonny's older brother.  The darkness of the tunnels and the narrator's being trapped there are foreshadowing the darkness and trapped conditions of the narrator's people, most particularly his brother Sonny, who bears the darkness and cage of his skin color, the poverty of his people, the darkness of his drug addiction, and the darkness of his music, the blues.