Below are two examples in the film versions of a story of how narration is used for dramatic effect.
THE SNAKE PIT (1948)
In this scene, Virginia Cunningham, who is played by Olivia deHavilland, is in the state mental hospital after suffering a nervous breakdown. Her husband is visiting her for the first time. She is sitting at a table waiting for him to bring over ice cream and coffee. She does not speak, but you hear her voice narrating the thoughts in her head. This film is based on the semi autobiographical book The Snake Pit by Mary Jane Ward.
THE SHAWSHANK REDEMPTION (1994)
In this scene, Red (Morgan Freeman) describes how he supposes that his friend Andy Dufresne (Tim Robbins) has escaped from prison. Red is not visible in the scene, we only hear his voice, but we see Andy. Red is the narrator in the original short story "Rita Hayworth and The Shawshank Redemption" from the Novella Different Seasons by Stephen King.
Narration in film is a part of its sound--voice over, dialogue, sound effects, and music combine into an auditory experience that enhances the visual one. When we read a story, the printed words take the place of the auditory, the narration is then spoken in our imagination, our minds, according to those words. The omission of sound can have as much of an impact in a story as its inclusion, hence the term "the sounds of silence."