As a function of character, flashbacks provide strands of memory that provide information that cannot otherwise be incorporated into a narrative. In "Sonny's Blues," James Baldwin makes very effective use of this literary tool as after learning of his brother's arrest for heroin use, the narrator recalls much of his family's past.
After the narrator picks up Sonny, he brings him to his house, a house not unlike the one in which he and Sonny grew up:
The moment Sonny and I started into the house I had the feeling that I was simply bringing him back into the danger he had almost died trying to escape.
This memory triggers the narrative's flashback to their childhood, especially when the narrator wants to hear that Sonny now is safe. He recalls that Sonny and his father were much alike, having "that same privacy." Then, the narrator remembers how his parents worried about the darkness and what the children must "endure." His mother has made him promise to watch out for Sonny so that he will not meet a fate similar to that of his uncle. He remembers his mother's admonition: "But you got to let him know you's there."
There are three other flashbacks that follow this one: The narrator recalls the first time that Sonny lived with him and he was going into the army. Sonny also wanted to join because he did not like school; the narrator urged him to stay in school, promising to help him when he returned, but noticing the shadows on Sonny's face. Then, in the third flashback, the narrator recalls how Sonny played the piano-- "playing for his life"--constantly while he lived with his wife. They have a fight, and Sonny left for months. When he returned, he told the narrator not to worry about him because he was dead as far as his family was concerned. Finally, the narrator recalls reading about "Sonny's trouble" the year that his daughter died. The narrator wrote to Sonny; his trouble made Sonny's real.
In a return to the present, the narrator continues,
One Saturday afternoon, when Sonny had been living with us...I found myself wandering aimlessly about the living room...trying to work of the courage to search Sonny's room.
Then Sonny returns from listening to a street Revival. Dramatically, the conversation and the triggered memories from the flashbacks enable the narrator to better understand Sonny.