Well, if you’ve written a novel about an individual who overcomes an alien abduction and goes on to become a champion marathoner before saving the human race when the aliens invade, you would not only market your book to lovers of science fiction, but also to survivors of alien abduction and to people who enjoy running marathons.
Writing a Book for Millions of Readers is Bad
It turns out, writing a book for millions of readers is a bad idea nowadays. Writing a book for the 12,863 individuals who claim to have been abducted by aliens in the last 5 years is a much better one. Now that the modern marvel known as the internet (not the inner netting of men’s swimming trunks) allows writers to market to such specific clumps of people via Facebook, etc., it makes much more sense to narrow the target audience.
Being the proactive individual I am, I asked myself, “Self, if it is good to identify niche audiences within my novel after I’ve written it, wouldn’t it be better to write my novel with specific niche audiences in mind from the beginning?”