I notice this a lot on Veeky Forums lately: people asking the most inept questions about great books, as if there were some deeper meaning in parts that were supposed to be funny, interesting, emotionally resonant, add layers of realism to the story, etc.
Do you want some allegorical essay brilliantly pinpointing how each of these chapters related to Melville's readings of Spinoza, Plato, and Shakespeare?
Will that make the book any more enjoyable to you? If I rationally over dissect and analyze it, if that's the only way you can care about it, what every part of it means, then hasn't the book already failed for you?
It's this trite attitude on Veeky Forums that the only purpose of a book is to make trite high-school tier essays about it talking about the political significance, the psychological significance, the philosophical significance, etc., of what certain parts "mean", why did the author write this part, that betrays the complete ineptitude of people on Veeky Forums.
What was the significance of the fucking book, if you want to ask why the cetology chapters, why those descriptions? Do you think this is an equation, with a reasonable part for everything? What if you took out the cetology, took out the details about working on a ship, took out the detailed descriptions and every attempt to add realism in the book? Then you wouldn't have a book.
Did the book satisfy you? If not, then who cares? No explanation could satisfy you, the book has already failed. It's as if us two were standing next to each other as friends and I were telling you a joke or a story and were trying to recreate the scenery of the place with a few details, and you wondered why I mentioned those details. They're as much part of the story as anything else.
In short, you are a fucking idiot
The point was to show how, despite all the scientific knowledge we have on whales, there's still holes in this scientific knowledge, and still forever a philosophical inscrutability in wondering why Moby Dick did/does what he does ... despite all the attempt to rationalize everything, to be encyclopedic, there's still something subtle, elusive, and irrational in life that perpetually escapes our attempts to put it in order. The overly detailed nature of the chapters is meant to contrast against what we DON'T know about life --- why we're alive, why we do anything, the existence of God, and everything. Dumbass.