Although Kurtz eventually becomes a corporeal character in Heart of Darkness, I think that Kurtz as just an idea is equally if not more powerful than the man himself. In the case of Marlow, the idea of Kurtz gave him a purpose in life, it allowed him to take advantage of an otherwise dismal situation, and it set him apart from the rest of the men who pursued a physical goal, ivory.
While the other men were consumed by their greed and their desire for ivory, Marlow became enticed by the idea of Kurtz, not what Kurtz was after. While the other men languished, Marlow stayed on his toes and more of an observer than an active participant.
Marlow's lesson applies to the real world as well. Searching for an idea, whether obtainable or unobtainable gives one's life purpose; often the corporeal form of the idea proves to be disappointing and cannot live up to the idea. Regardless of whether or not a physical or real thing can be acheived by pursuing an idea, the purpose that the idea gives is what makes the idea valuable.