The frame narrative of Rumble Fish occurs with Rusty James telling his old friend, Steve Hays, the details of his story. The novel begins long after the main events of the story actually take place, with Rusty James and Steve Hays meeting on the beach. As they begin to reminisce on their shared past, the novel transitions into its frame narrative, stepping back five years into the past.
A frame narrative is a form of literary structure in which one story is essentially contained within another story; frequently, as in Rumble Fish, the details of the inner story are told directly by a character in the outer story.
There are several different reasons why S. E. Hinton would employ a frame narrative in Rumble Fish. One possibility is that by creating the frame narrative, Hinton is able to explicitly show that Rusty James considers the events of the story to be significant. Since he is retelling the story, it can be surmised that each detail is something he considers to be important.
Another accomplishment made by using the frame narrative is that it immediately sets the protagonist, Rusty James, at a point in the future that the reader has to work to get to. It is made clear that Rusty James has recently left the reformatory. By then jumping back five years, the reader is left to slowly piece together why and how Rusty James spent time in the reformatory.