The purpose there might be for Marlow's long wait to get his steamboat fixed and on the river is to explore and observe what is going on and what's happening. This symbolizes mystery because Marlow still doesn't really know what's going on around him and this is a chance to find out.

i think marlow's wait will make him get more curious before he fixes his steam boat. that empty space on the map can only be acheived by that steamboat that he is building. but if he cant fix that steamboat fast, he wouldnt be able to see whats over there.

I think that the steamboat symbolizes either staying here and learn more or move and leave it behind