Knowing your vision will help you layout your book in the best way for that audience. Even a cookbook can be put together in many different ways depending on who you want to reach. For example, are you trying to reach out to young couples with quick and easy recipes? If so, you might put together plans in smaller portions so they can use the leftover ingredients for the next recipe. If your cookbook is geared more to busy moms with kids, you might decide to group the meals in Breakfast, Lunch, and Dinner days so she can basically "read" the recipes right through.
With my novels, my audience is those who love mystery and adventure, but who also want good clean fun. My books don't get too scary, don't have murder, don't have sex scenes, etc. I want anyone to be able to read my books and enjoy without worrying about something "going too far." My "bad" guys even have a heart sometimes.
Some might ask, can you vision change? And to that I would say definitely!
When I started writing, I thought it would be really fun to have my book classified as a Christian Fiction. That would be a pretty cool category to be a part of. But what I found was that I didn't enjoy reading too many Christian Fiction books in a row. Why not? Because it always seemed like the main characters were not my main focus...that might sound a bit weird, but I like the adventure, mystery, and new friends that my main characters meet through the novels. Some even come back in future books, which is something I do intentionally. I think one reason why I do this is because each person really is just a single character in the big picture (or movie) we call life. Each of us might not have a huge impact on the world and how the whole world is going, but we do have control over which direction our little slice of the world is going, even if that little slice only includes you.
My main character Chyli, and her friends, take what they have and enjoy doing and share with others. They reach out and help others who might not feel that anyone notices them. They stick their necks out, even to assist those they hardly know. They see a need and they go do what they can to help. I want to see this more in the real world.
My books are fiction, but I hope and pray they inspire every day people to look outside themselves and make a difference.