In this passage, Ishmael wanders around New Bedford and gives us a description of this town and its various characteristics and foibles. What is interesting about it is the way in which New Bedford is compared in the passage you specify to the coast of Labrador. The passage asks the question why is it that New Bedford is such a lovely place to live whereas the coast of Labrador, and from where all the wealth that New Bedford exhibits comes from. The following answer is given:
Go and gaze upon the iron emblematical harpoons round yonder lofty mansion, and your question will be answered. Yes; all these brave houses and flowery gardens came from the Atlantic, Pacific, and Indian oceans. One and all, they were harpooned and dragged up hither from the bottom of the sea.
Note how the whaling metaphor of harpooning and dragging is used to describe the origin of New Bedford's wealth. This passage helps us to realise the way that certain industries have been responsible for the wealth of certain locations. New Bedford's prosperity and well-being is a direct result of whaling and therefore all the citizens of New Bedford are involved in whaling, at least by extension. In the same way, to make the link to us, every location owes its wealth and prosperity to certain trades that have made that city into the place that it is today. This becomes rather disturbing if we think of the role that trades such as slavery have had on the creation of cities such as Bristol in England or other similar locations.