how does capote use flashback?

As is true in any writing flashback helps to develop individual characterization and help the reader understand the background of the characters. Specific characteristics of personalities come out of someone's childhood - for example, Perry's childhood somehow influenced how he acted. It can also fill in holes in the understanding of the story and the exposition that leads to the rising actions and the climax.

flashback help you to recall what happens in the past

flashback help you to recall what happens in the past