A good way to consider why authors begin their texts the way they do is to compare and contrast the ending. Let us remember that this excellent story opens with the discovery of the narrator concerning his brother's arrest for drug possession and selling heroin. Key to note, however, is the way that this discovery makes the narrator feel trapped and surrounded by darkness:
I read about it in the paper, in the subway, on my way to work. I read it, and I couldn't believe it, and I read it again. Then perhaps I just stared at it, at the newsprint spelling out his name, spelling out the story. I stared at it in the swinging lights of the subway car, and in the faces and bodies of the people, and in my own face, trapped in the darkness which roared outside.
The story thus begins with the discovery of the narrator's brother's arrest and the way that this piece of information oppresses the narrator and makes him feel trapped. The rest of the story tells us, in a non-chronological form, the gradual journey of the narrator and his brother towards wholeness and unity once again. The reason why the story begins as it does is to show the growth in understanding of the narrator and how he comes to appreciate and learn from his brother and the music that is so important to him. Note his attitude towards Sonny and jazz changes by the end of the story:
Freedom lurked around us and I understood, at last, that he could help us to be free if we would listen, that he would never be free until we did. Yet, there was no battle in his face now, I heard what he had gone through, and would continue to go through until he came to rest in earth.
The narrator has moved from feeling "trapped" at the beginning of the story to being aware of the "freedom" that lies around him and the way that jazz music allows its listeners to grasp that freedom. The way the story begins thus adds a dramatic contrast to the progression and development of the narrator.