At the first couple of writer’s conferences I attended I asked authors what to call my first book. Mystery? Thriller? Suspense? I was told more than once that “genre” was a publisher’s invention, of value only to the marketing team.

I’ve since learned to view that fact from the other side. Yes, it’s a marketing tool, in that it helps sell books by letting readers know if they want what you write. Genre is a way to help your natural audience find your book.
Each genre has specific conventions that readers expect. If you write genre fiction, it pays to meet those reader expectations.  Sure, the best known authors wander far from those expectations but if you’re starting out it’s a lot easier to build a readership by writing a book that fits exactly into your genre. Hopefully you’re writing what you love to read, which should mean you know what those conventions are.
I write private eye mysteries. I know the kind of character my readers want, a detective who’s dedicated to his client and to finding the truth. These detectives live in a dark, unforgiving world and will get violent when they need to. They’ll get knocked out but seldom show the results of repeated concussions. And in the end, the bad guy will ALWAYS get punished.
On the other hand, Penny Clover Petersen writes cozy mysteries. Readers expect her amateur sleuths to be, and meet, quirky characters. There will be no blood or gore, and little profanity (unless it lends itself to humor.) She stays within those lines but still manages to create unique and unforgettable stories.
Each genre has dedicated fans who read several books every month. Staying in your genre gives you a guaranteed audience, assuming you write good stories with interesting characters.  When I hear a writer say her story has something for everyone, or that it will appeal to every reader, what I hear is “I don’t know who my audience is.” And if a reader who picks up your book isn’t sure if it’s focus is on being a paranormal fantasy or historical romance or a traditional whodunnit, they, will most likely put it back down and look elsewhere.
In my experience, all the plusses of sticking to a clear genre are magnified if you decide to write a series. After the first book, everything is established. Readers are already familiar with the setting, they know your main character, and the secondary characters are expected. And in terms of sales, it’s easier for readers to find your follow-on books because series take up more space online. And once you’ve hooked them, they’ll search out your books.
So remember – fiction genres are your friend! Stick with them.