When you want your point to stand there are different ways to be heard. You can shout it, or you can merely repeat it many times until it's left ringing in everyone's ears. Repetition is the key. To make constant emphasis on something can be annoying, but if it is annoying it means the point has been made, and everyone is feed up with that particular idea. That is exactly what Joseph Conrad does in a subtle way in Heart of Darkness.
We must take into account that this is a frame story. It isn't supposed to be novel, but rather a tale told casually to some friends in a boat waiting for the tide to rise. Apart from the fact that syntax is very different when we talk than when we write, our expressions are different too. Written expressions are different than oral expressions. Orally we tend to repeat the same thing various times when we get anxious or our emotions heightened. As the tone of voice changes, the feeling of monotony changes too, but when it's written we just don't picture the characters shouting the lines, but we rather calmly read them. It's different to give expression and emotion to a written work, than to an oral work. Marlow's repetitive narrative adds uneasiness to the flow of the novel. It could mean that Marlow is panting, and fidgeting while telling his story. While Marlow says there were "Trees, Trees, millions of trees, massive, immense, running up high" (Pg. 63), one could simply write "there were many trees" to make it less wordy. It sounds as if he were overwhelmed at the moment due to the recap of his eventful journey, and not woried at all abour wordiness and being consice.
When we believe something is important in our minds we tend to repeat it or mention it a lot as we are constantly thinking about it. We are quite aware that Marlow's experience had an emotional effect on him, and he most likely remembers the past with emotion and it is seen through his tone and diction. This ads intimacy and emotion to the tone of the novel, making it even more intriguing.

