Economics of the Congeries
This was certainly the case in 1934, when William Faulkner began writing a series of stories about Bayard Sartoris and his slave companion Ringo for the Saturday Evening Post. Faulkner was published by Random House but made little money from book publication of his work; a single magazine sale to a major slick could pay more than a novel advance. Faulkner liked to write short stories and later rework groups of them into novels, but you can see how writing a novel that would definitely yield magazine money (as, "Absalom, Absalom!" and "The Wild Palms," the novels he wrote immediately before and after "The Unvanquished," did not) would appeal to him.
Ditto, interestingly, for Ernest Hemingway, whose books were not quite as successful in the mid-thirties as they had been at the decade's beginning, but who was being paid a fortune by Esquire for anything he'd write for them. Soon enough he was writing the stories that went into "To Have and Have Not," which people (perhaps with the movie version lodged in their memories) do not recall as an example of this kind of novel.
Bernard Wolfe was another well-regarded novelist (he is best remembered for "Limbo") who made more money for his magazine fiction, and soon wrote a congeries novel. The novels he published in the fifties -- with Random House, Knopf, and Scribners -- made him little money, and the last of them ("The Great Prince Died," about the last days of Trotsky), failed to sell to paperback. But he had meanwhile become a regular contributor to Playboy, and he wrote his next novel, "Come On Out, Daddy," (1961) as a series of long stories for the magazine. I suspect that he made a lot more money with that project than any other, for in the mid-sixties he began two more such series. (They never appeared in book form; around that time Playboy got a new fiction editor who got rid of their regular writers -- of Wolfe's and Irwin Shaw's generation -- for a younger crowd, and Wolfe lost that market.)
And in the early seventies, when several of the finest SF congeries novels appeared, the same conditions obtained. The original anthology field paid four or five cents per word, which the best-paying magazines (Galaxy, mostly) would match for preferred contributors. Even established writers got novel advances of around $3,000 -- if there were paperback and book club sales, the author would eventually see a few times that, but you couldn't get a hardcover advance that would guarantee it. This began to change very rapidly around 1973, but for about half a dozen years before that, writing for Orbit or New Dimensions was financially a good deal.
I don't want to overstate this, but it is interesting. (The novel market was hugely more prestigious and renumerative relative to short fiction in the eighties and nineties, during which time, indeed, there were relatively few congeries novels in SF.)