Why is Heart of Darkness structured into three parts? How is the organization of the storytelling significant?

Heart of Darkness is a novella, told in flashback, which chronicles a man's journey and return from literal and figurative darkness. It's three-part structure follows the narrator, Marlow, as he leaves London, England for a job deep in the Congo, meets the enigmatic Kurtz there, and returns to England a changed person. Through this structure, the novel explores the effects imperialism has on both the domineering country and the native persons.
The first part of the journey follows Marlow as he prepares for his job. He travels to the Outer Station, where he must remain, as the boat he needs is literally under water. Here he notices the inefficiency and waste of the Englishmen. He also notices the cruelty the white men have for the natives. Marlow is surprised at their complete ineptitude at preparing a the boat for the journey, but he does have a chance to hear rumors about Kurtz, the imperialist leader of the inner station. Rumors have it that Kurtz has become crazy in the Congo and must be removed.
The second part of the story takes place as Marlow's journey to the Inner Station is finally underway. As they near the inner station, the boat comes under attack. Marlow learns later that Kurtz himself ordered the natives to attack the boat. He had become a "native" himself and did not want to return to civilization. He and Marlow speak of the "hells" of the jungle and the problems with the ivory trade itself. When the Englishmen begin to remove Kurtz, the natives revolt, showing their dedication to him. Although Marlow must bring the dying Kurtz back to civilization, he does strongly question the morality of the ivory trade and imperialism as a whole.
The last part of the journey brings Marlow home and returns him to his position as a storyteller years later. Although Kurtz dies on the journey back, Marlow has internalized his experience with him. He seeks out Kurtz's "intended" and lies to her about Kurtz's dedication to her. We can tell by Marlow's desire to tell this story that this experience has changed his mind about life in England.