As an author, it’s extremely important that you truly know your reader. You should know their likes, dislikes, and what really makes them tick. If you know your reader, then you’ll be able to write books they love.
Don’t be the author who just writes whatever they want to write without giving a thought to the reader at the other end of things. You might think you know what they want, but it never hurts to do at least the bare minimum of research. In most cases, it makes a huge difference.
The only time where you wouldn’t have to gather much insight into your audience is if you’re already part of your audience and you interact with other members of that audience on a daily basis. For example, I don’t have to spend the time actively researching the at-home entrepreneur niche because that’s how I spend my day, in and out. I manage two extremely active communities in this niche and so I guess you could say I gathered my data over time.