The frame story, in which the world-weary Marlow tells his story to a group of sailors, creates a disconnect between the essential story itself and the reader. The unnamed narrator only interrupts occasionally, never with a critical opinion, just to describe Marlow, and it is almost as if Marlow is speaking directly to the reader. Conrad could have written the story in second-person -- "You listen as Marlow tells his story" -- but chose to add the unnamed narrator for reasons of his own.
"And this also," said Marlow suddenly, "has been one of the dark places of the earth."
He was the only man of us who still "followed the sea." The worst that could be said of him was that he did not represent his class.
(Conrad, Heart of Darkness, gutenberg.org)
One effect of this style is that the world of the story seems larger than just Marlow's experience. Had Marlow himself been narrating directly, his trauma would have been directly aimed at the reader, not at a specific audience. Instead, it seems that there is a vital and continuing world in the novel outside of Marlow's story; the sense is that the world has continued and will continue long after Marlow has passed from it. Another effect is to remove some of the tension from the worst scenes; Marlow has clearly survived, or he would not be relating his story to others. The reader can relax a little -- but not too much -- knowing that whatever else happens, Marlow gets out alive.