Why does John Knowles write A Separate Peace as a flashback?
The purpose of the flashback is to lend perspective on the story. Now that Gene is an adult, he revisits Devon and reminisces about his schoolboy days. He recounts the story from the perspective of his older, wiser self which affords him the distance to see the incident for what it was and tell it in a more mature and level headed way. If you take away the flashback, either the story would have to be told in 3rd person, robbing the reader of the firsthand insight the narrator is able to provide or it would be told by a more emtional Gene retelling his recent experience in a more emotional and/or less candid way.