The purpose of that chapter, the narrator says, is "to attend to a matter almost indispensable to a thorough appreciative understanding of the more special leviathanic revelations and allusions of all sorts which are to follow."
[1] Although writing a work of fiction, Melville included extensive material that presents the properties of whales in a seemingly scientific form.
The detailed descriptions are a digression from the story-line, but critics argue that their objectivity and encyclopedic form balance the spiritual elements of the novel and ground its cosmic speculations.
The cetological chapters do add variety and give readers information that helps them understand the story, but Melville also has thematic and aesthetic purposes.
Critics justify and even praise the sections for keeping the metaphysical and spiritual meanings in the novel anchored to matter-of-fact reality and balance the extraordinary with the ordinary.
The extensive descriptions show that the starting point for the “cosmic and spiritual is earthly and physical” and give the novel what one critic calls the “illusion of objectivity and the effect of a wide view of life.”[3] Ishmael asserts in the novel that the whale is a "spouting fish with a horizontal tail".