On pages 99-104, why does the voice of the first narrator intervene at that point in the narrative?

I hope this makes sense,

So what’s the point of this nameless narrator? This has much to do with the setting discussion. Because we have another narrator, we can stop Marlow’s story and hear commentary on the Thames River and its surroundings. We also get those great little lines about Marlow’s voice so we can parallel him to Kurtz. In short, the nameless narrator is an opportunity for more commentary, more connections, and more flexing of Conrad’s literary muscles.

But actually, there’s more to this question. As it turns out, Conrad slips in a little answer right in the first few pages when he describes Marlow’s opinions on story-telling. Here it is:

"[…] to him the meaning of an episode was not inside like a kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze, in the likeness of one of those misty halos that sometimes are made visible by the spectral illumination of moonshine."

If the meaning of this story is similarly on the outside, then we need to be outside this story (i.e., on the Thames) to get at it. This supports the theory that the point of Heart of Darkness is to draw parallels between the Thames and Congo Rivers, Europe and Africa, white Europeans and black Africans, etc. After all, Conrad flat out says that we can’t be in the story in Africa to get the meaning – rather, that the meaning envelopes the story in the narrative haze that is the frame-story. So, basically, it’s a good thing that we have that other narrator handy.