"Sonny's Blues" begins in media res. What does Baldwin achieve by beginning the story as he does?

"I had kept it outside me for a long time," Baldwin's
narrator says in the fourth paragraph of his story, "Sonny's Blues." The news of his
brother Sonny's arrest resurrects what the narrator has tried to bury in his heart. 
But, with this resurrection in media res there is also
the re-emergence of the memory of his daughter, and of his childhood with Sonny,
memories which play an integral part in the narrator's later understanding of the
darkness of his brother's life.

With first person point of
view, Baldwin's story achieves much more clarity to Sonny's condition by the narrator's
providing his history as well as his brother's.  For, in the flashbacks the narrator
recalls events that fuse the past, present, and future as parallels are drawn between
Sonny and their father, between the boys of Harlem then and the boys of Harlem now. 
Like a musical piece, these images mingle with others of darkness and of sound.  Sonny
and his brother watch a street revival, and likewise feel a revival of brotherly love. 
Clearly, this immersion in the middle allows both the past and the future to be brought
together in Baldwin's story, thus enabling the narrator to better appreciate the trouble
of Sonny's soul, his blues, and realize that what Sonny feels, he feels:  "My trouble
made his real.....And his triumph, when he triumphs, is
ours."

At the end of the story, the narrator pulls in the
reader, as well, with his singleness of
theme, saying,

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For, while the tale of how we suffer, and how we
are delighted, and how we may triumph is never new, it always must be heard.  There
isn't any other tale to tell, it's the only light we've got in all this
darkness.

The music has
helped Sonny express himself and take control and avoid his suffering, just as the blues
can help everyone be true--have the glow above their heads--to what they
are.