Many narrators jump around for a variety of reasons. The most typical reason I see is to give the reader some background knowledge that is required to understand the current situation. Another very important reason that author's do this is to develop character. I think Conrad does this in reference to Kurtz throughout the book.
We hear about Kurtz from a variety of perspectives well before we actually see him. These multiple perspectives serve to force readers to make their own choice about his motivations and morality. Marlow seems to have an amazing admiration for him. Conrad, the author, may not really have that same passion.
An example of this occurs when the unnamed storyteller reports Marlow telling of the time in the beginning of the second section what he was over-hearing about Kurtz. The uncle and the manager were discussing Kurtz' effectiveness and the manager seems to have a distaste for Kurtz. It is as if Marlow is trying to determine if Kurtz is going to be purposely taken out, or if his sickness is taking the life from him. This example serves to create intrigue and curiosity in Marlow, and likewise the readers. We are continuing to try to determine if Kurtz is inherently good or evil. The closer we get to him, the more we find the latter to be true. Thus, Conrad builds the concept as time goes on, but every once in a while a quick jump in time is necessary to go back or forward to a moment about Kurtz to make us wonder. The best books keep the reader guessing. Conrad successfully employs this strategy.