The simplest reason that Melville included such digressions is that they were common in period novels. To expand on that a bit more, in a society before television, radio, or other broadcast media, books and newspapers played multiple roles. Novels like Moby Dick gave descriptions of these topics to educate readers, and to entertain them, as a travelogue might.
They also build the narrator's authority. Rather than just being a whaler/sailor, who has a single profession, he's someone who can discourse on many topics. This lends weight to any of his observations about character in the novel.