A question related to “Why do you write?” but slightly easier to answer, is “Why do you write novels?”
Because we can’t write any shorter.
I read numerous writing advice where they say, “Start off with small steps. Start off writing short stories, and then graduate up to novels.” Sound advice but I, for one, have never been able to do it. Every short story story we have ever written is sitting in our story ideas file, waiting to be turned into a full-blown novel. The characters in these shorter works have stories just waiting to be told, worlds we need to write about, ideas we want to explore. Way too much information to go into a short story.
It’s a pity, because I think becoming known as a writer of short fiction would definitely help us to sell novels—both to an agent and to the general public.
From my own personal experience, I will buy a novel written by someone whose short works I have enjoyed. Having read Connie Willis’ Firewatch and A Letter From the Clearys (okay, Firewatch was a novella), I went out and bought her novel, Lincoln’s Dreams. True Names (another novella) introduced me to Vernor Vinge’s writing.
It’s not just short stories. Any writing will do. I bought Nancy Kress’ books based on her monthly column in Writers’ Digest. I admired her writing there, liked what she wrote, so when I came across Beggars in Spain in a bookshop one day I bought that.
Outside of this, I tend to discover new writers at the library, rather than buy books on spec.
If you can write short stories then do so. Not only does it hone your writing skills, but you have a much better chance of selling a shorter work than a full blown novel, and the credits you get from that short story sale may just help you sell your novel.