It goes with the human need to classify or name everything. I believe people are more comfortable if they can attach a genre or a form to any given creative work, and it aids marketing decisions. If the book has been labelled a romance by its publishers, it may not be a compelling sell to someone who prefers science fiction or fantasy, regardless of the book’s innate merits. And the opposite holds true. However, there are works that can be called “unclassifiable”. Thomas Pynchon Jr’s The Crying of Lot 49 made critic and editor David Pringle’s hundred best books of fantasy despite being labelled a post-modernist work by many. It could be argued it is a whodunit as well.
Another example of the unclassifiable would be Mervyn Peake’s Gormenghast series of books. Despite being set nowhere we can relate to or know about, there’s nothing overtly fantastic or supernatural in the books. Everything that happens in these ostensible fantasies could happen in downtown Lismore. As Pringle says though, they’re “undeniably strange” but they avoid easy categorisation. Sometimes the artist themselves dislike categorisation of their works. American author Don DeLillo stated in an interview that he doesn’t view his work as post-modernist or any other genre and as he says: “I’d prefer not to be labelled. I’m a novelist, period. An American novelist.”
So, an argument could be made that categorisation is a tool solely there to help people make buying choices.
DePietro, T 2005, Conversations with Don DeLillo, University Press of Mississippi, Jackson. p. 115
Pringle, D 1988, Modern Fantasy: The 100 Best Novels, An English-Language Selection, 1946-1987, Grafton, London