"There is no way to tell [Kurtz's] story without telling my own," says Captain Benjamin Willard. "And if his story is really a confession, then so is mine."
Sometimes people criticize voiceovers, arguing that movies should "show and not tell." Fair point. Really famous voiceovers, though—like those in Sunset Boulevard, Taxi Driver, and Apocalypse Now—show that this technique can work.
Capt. Willard functions as the movie's first-person narrator. He's our surrogate: we see what he sees, since he mainly acts as an observer. The movie never ranges too far outside Willard's range of knowledge; we don't know more than he does, and the voiceover helps us understand his actions in the film.