What is gained by making the film a flashback as opposed to the way it is presented in the book?

As far as I know, the term flashback in the fields of cinema and literature is treated as a brief return to the events of the past, look back. In the movie, during the term may be interrupted in the frames that are in the mind of the hero - his memory, reasoning, sensation in the past. These frames can be repeated in a film already displayed. This method of storytelling can even create a basic idea of the composition. In the literature flashback allows us to understand the thoughts of the hero, to explain his actions.