When adapting a book into a movie, voiceover is a reliable fallback.
It’s an easy means of retaining as much as possible of what originally worked about the writing, assumably the reason anyone would be interested in translating the novel to the screen in the first place. Obviously the creative team still needs to visualize the text cinematically — literary storytelling is not the same as cinematic storytelling — but voiceover eases the burden by injecting the author’s literal voice into the audience’s ears.
Which is why I was so taken by The Quiet Girl forgoing voiceover. The narration of Claire Keegan’s short story can be relocated into dialogue only so much (which is one way for cinema to embody a textual medium). But for the most part, the movie uses Keegan’s words as a guide to devise its own cinematic aesthetics that feel and look and operate true to the spirit of her writing, all while minimizing verbatim mooching.