Flashback is important in “Sonny’s Blues” because it provides a context for the events of the story. Knowing what happened in the past informs us of the present.
Time in the story goes back and forth between the past and the present. For example, when the narrator hears his students, they remind him of his brother at that age.
They were filled with rage. All they really knew were two darknesses, the darkness of their lives, which was now closing in on them…
The flasbacks are also relevant to the theme of the story, which is partially that you need to try to understand people before you can judge them. The narrator remembers his mother telling him to look out for his brother. He feels bad because he does not think he has done that.
"You got to hold on to your brother," she said, "and don't let him fall, no matter what it looks like is happening to him and no matter how evil you gets with him.
When he does try to get to know his brother, he sees that he is not really the man he thought he was. The seven years’ difference has resulted in the two of them not really knowing each other. For example, when Sonny hears the woman singing and confides in his brother about why he used heroin, this helps the narrator understand his brother.
"It makes you feel-in control. Sometimes you've got to have that feeling."
The flashbacks in the story pull the reader along through the narrator’s thought processes. Through them, we understand not just Sonny but the narrator. We realize that neither of them has really given the other a chance.
Families are made up of memories. All of us have experiences with our family members in the past that shape how we look at them. In this story, we realize that the past is just as important as the present to understanding someone.