Heart of Darkness is structured as a story-within-a-story, or a story told by someone to someone else. The narrator, who is unnamed, hears the story second-hand from Marlow, who tells it while their ship is at anchor. This lends the story a certain amount of verisimilitude, or realism; the narrator is not professing the truth of the story himself, but letting that burden fall on Marlow.
We looked on, waiting patiently -- there was nothing else to do till the end of the flood; but it was only after a long silence, when he said, in a hesitating voice, "I suppose you fellows remember I did once turn fresh-water sailor for a bit," that we knew we were fated, before the ebb began to run, to hear about one of Marlow's inconclusive experiences.
(Conrad, Heart of Darkness, eNotes eText)
Another effect of the frame-story is to lessen the impact of the atrocities that Kurtz and others have committed. Since the narrator is hearing the story from Marlow, the reader is two levels removed from the personal nature of Marlow's discoveries. This allows a slightly more objective examination of the motivations behind Kurtz's actions, as well as of the other members of the ivory company. By telling rather than showing -- the opposite of most authorial advice -- the reader is invited to focus on the meaning of the work instead of the images and text itself.