The use goes back a long way. In a very quick search I found it on Sinclair Lewis’ It Can’t Happen Here, from 1935, and Ayn Rand’s The Fountainhead, from 1948. It’s not on early Hemingway, but it is on his later books. I’d say it stopped seeming new and different after WWII and became the norm by the 1960s.
The industry uses it for two reasons. One is prosaic. If you, the reader or potential buyer, pick up a book, you want to know immediately what it is. A box of cereal and a box of laundry detergent may have the same dimensions and you may want both on your grocery list, but you don’t want to confuse them. Titles can be ambiguous or straight out misleading. The two above could easily be applied to a non-fiction book.
A novel is also different from a collection of short stories by one author, or an anthology of stories by many authors, or two novellas bound together, or humorous essays, or a spoof, or an outright parody.
The second reason is more insidious. “A Novel” is code. In the publishing industry, “A Novel” always means a literary novel. Literary novels have been traditionally the highest level of publishing. “A Novel” is like USDA Prime beef. Choice and Select are mostly what you find in stores, and the grading goes all the way down to Canner. Publishing has a similar, if unwritten, grading.
The industry has always been split between top-level literary novels and regular novels that typically sold a lot more copies. Neither Gone with the Wind nor Payton Place had the designation. James Patterson doesn’t get it today. There aren’t many literary trilogies, either, and none would ever rise to that level. However, each of John Updike’s Rabbit books is labeled “A Novel.”
Just as important as separating literary novels from bestsellers is distancing “real” novels from genre novels. Only a very few genre novels ever get the “A Novel” designation, and that generally appears only after the writer has ascended into the rarefied atmosphere of critical acceptance. Raymond Chandler didn’t get it until 1954’s The Long Goodbye. Ursula K. Le Guin didn’t get it on The Left Hand of Darkness but did on The Dispossesed.
Most fiction today is published without the “A Novel” designation. Nevertheless, you may indeed be seeing it more often. The industry is panicking, like all other print industries. It’s trying to make certain books prestigious, “must” reads, that will be talked about online. Those books and those authors will certainly be favored on their front covers. You’ll probably read more reviews of them in more places as the publisher pushes them into the spotlight. And then you’ll forget they ever existed, because most of them will never sell and be remaindered. Because readers of “A Novel” are a tiny minority of readers.