The first person you have to write for is yourself but there is someone else you must keep in mind while writing, your audience. If you don't know your audience and keep them in mind, chances are they won't connect well with your novel. Genre can help you narrow it down and identify who they are.
If you pick up a young adult novel you'll notice they have a particular tone to them, the tone of a teenager. The language is different, the thought process is different, and the plots of such books are focused around what would interest a teenager. A woman's literature novel will read with the tone of woman. It will play to women's interests by focusing on subject matter they want to read about. There is often a more emotional feel to these novels. Thrillers tend to read with an exciting edge that appeals to men. Men have their own language to a point and operate more on action and literal thinking, this is reflected in novels written for them.
There are always exceptions but more often than not you'll notice a different tone for books that are meant for men, women, or children. If you are writing something less obvious like fantasy, sci-fi, or something that appeals to both genders then the best thing you can do is make sure your tone is gender neutral so it doesn't exclude anyone. A good balance of action and plot development is another good way to please a general audience. Think of The Notebook, Jonah Hex, Harry Potter, or The Lord Of The Rings. Each is very distinctive and appeals to a particular audience, save for the last which spans across all ages and both genders. For research and to get a better idea of this read a few books outside of your genre.
For more advice on the subject check out this post by Writer's Digest.