I imagine that back in the days when it was common (indeed, customary) for novels to be built on the conceit of being collections of found correspondence, or autobiographies, the subtitle was extremely helpful to the casual browser. (In one of the Emily books, there's a bit where Emily reads The History of Henry Esmond despite being forbidden to read novels because her Aunt Ruth is fooled by the word "history" in the title.)
Even these days, novels often have titles that are enormously misleading; A Short History of Tractors in Ukrainian, for instance.
See, it's books like that one that cause people like me confusion when there isn't something on the cover to indicate that a book is or is not fiction. I can't remember the number of times I've seen some display of books in a store, and picked one up only to wonder is this fiction? If it doesn't say "novel" somewhere prominent I usually conclude that the book is in fact non-fiction.
I think it's primarily a quick and easy guide for bookstore employees who need to know if a book is fiction or non-fiction for shelving purposes, without having to to stop and read the DJ flap. Especially convenient for authors such as Richard Clarke or Newt Gingrich, who have written both fiction and non-fiction.
In the case of Dave Barry's novels, it served as a warning that they were significantly different fare than Dave Barry fans might normally have expected from him, given his publishing record.
"Title: a novel" is a tad retro, but I can't see as it's worth commenting on as long as the work doesn't turn out to be an essay, picaresque, collection, concatenation, fix-up, fragment, or other non-novel.
And I'd give a pass to a fix-up if the author successfully made me feel that the previously-published portions had been fragments of a longer work all along.
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