Clearly you haven’t read Neal Stephenson (Quicksilver, Cryptonomicon).
I don’t think there is one specific answer to this question. In some cases, like Melville, the divergence is for a reason; Melville is establishing a metaphor (albeit one that may be oblique to an audience that doesn’t know his intent.) With someone like Joyce, divergences were just a part of his style; indeed, he was little interested in plot or traditional character development. Burroughs went even further, dismissing any kind of linearity or coherence in plot; his postmodernist prose was trying to recreate the kind of confusion and disorientation that he experienced. And Faulkner…well, he was interested in seeing just how many adjectives, adverbs, and recursive prepositions he could stuff into a sentance. Sometimes you start to wonder if he was interested in the nominal topic at all.
I’d say often enough this is done to create a mood, or describe a time period/manner of behavior/technology/et cetera that the reader may not be familiar with. Other times it is to break the tension, or create alternate plotlines that may not be readily apparent. And sometimes it is done just because, well, because the author was just interested in the topic, assumed the reader would be to, and had a lazy, careless, or cowed editor that refused to trim it out. When this is done poorly it comes off like bad voice-over use in a movie; unnecessarily expository or insultingly patronizing. When it is done well–the best example I can think of are the Guide entries in The Hitchhiker’s Guide To The Galaxy–it works to amuse the reader, inform on the topic at hand, or presage the next plot development.
Not to suggest that every writer who does this has some deeper meaning or is a champion of his or her art, but becoming a more accomplished reader (i.e. capable of identifying the fundamental themes, symbolism, metaphor, et cetera) in literature often involves moving past an obsession with linear plotting and “logical” character development. The often-lofted complaint at, say, Heller’s Catch-22 is that a) nothing happens, and b) it keeps happening over and over again; while I think it’s a bit of a stretch to say that nothing happens, the repetitiveness, the nonlinear and disorienting chronology, and the general sense of confusion and lack of movement is quite deliberate on Heller’s part; it’s a stylistic choice which draws the reader into Yossarian’s plight, the inescapable, utterly maddening tautology of Catch-22. (“That’s a good catch, that Catch-22.” “It’s the best there is.”) If the text is frusturating to read then you are getting the point; the sense of frustration of a frightened, bewildered bombadier who just wants to complete his requirements and get the hell out, for whom abstract appeals to patriotism and justice have less draw than his perfectly rational paranoid delusion that everyone is trying to kill him.
Ditto with just about everything written by Thomas Pynchon (Gravity’s Rainbow), and even earlier, more plot-driven writers like John Steinbeck do this considerably; in The Grapes of Wrath, for instance, Steinbeck diverges several times to complain of the banks, the gentlemen farmers who exploit the immigrant Okies, the police and politicians, et cetera. He was taken to task many times for his blunt interjection of social criticism into the text, rather than moralizing within the text of the story, a la Upton Sinclair or H.G. Wells, but in fact it (IMHO) it makes his stories stronger to seperate the social criticism which underlies them from the story itself; it prevents him from having to insert a bald morality play into the story (as he did in some of his lesser stories, like Tortilla Flat or Cannery Row) and letting the action take place in an unforced manner.
This is not to say that a “great novel” has to have all kinds of elaborate subtext, symbolism, and so forth in order to be great; Harper Lee’s To Kill A Mockingbird is a straightforward semiautobiographical reminiscence of Lee’s childhood. It’s a powerful story which neither needs or would be served by any artificially-inserted metaphor or moral, and is all the more thematically complex for its narrative simplicity; a simple aside about her brother hacking down the flowers of a foul-mannered, bigotted neighbor, and the pennance he is required to pay to make up for his actions says volumes without having to force any symbolism at all.
I’ve not read The Hunchback of Notre Dame, and it’s not even on the extensive backlog of books I have to read, so I can’t speak to the specifics, but I’m guessing that Hugo probably had some purpose in mind when drafting the divergence in question. What it means…well, that’s up to you.
Stranger