What is the importance of the frame story in Heart of Darkness?
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.
At the beginning of Heart of Darkness, an unnamed narrator listens to Marlow aboard a ship in calm waters on the Thames in England, far from Africa. The importance of the frame story is that it allows the reader to experience Marlow's journey along with the narrator. As Marlow tells the tale that convinced him of the evil of imperialism, the narrator--and, by extension, the reader--go along with him and experience Marlow's sense of growing alienation and disenchantment with Kurtz and with European imperialism in Africa.
The narrator speaks about the bonds of people on the sea when he says the following:
Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together through long periods of separation, it had the effect of making us tolerant of each other’s yarns—and even convictions.
This introduction asks the reader to be tolerant of Marlow and to be open minded as he speaks of his experiences in Africa. Therefore, the frame story invites the reader to be an open-minded participant in listening to Marlow's story. The result is that the reader, like the narrator and Marlow, can begin to question the wisdom of formerly accepted truths like the benefits of imperialism.