Knowing who might enjoy the thing you're putting out into the world is a crucial, often overlooked, part of your campaign. It may seem obvious at first that the people that want to see your film, read your book, or watch your play would be a person like you, but it's important to take the time to find out who those people are. What else do they like? What organizations are they a part of?
By taking the time to think about who your audience might be, you're also helping to focus the goal of your project.