A book genre is a category that tells readers what kind of experience to expect. A reader who picks up a mystery wants a puzzle with a fair solution. A reader who picks up a self-help book wants something practical to try. Genre is a promise, and a book that breaks it loses the reader.
It also decides where your book sits online and in stores. Retailers and libraries sort titles using shared subject codes such as the BISAC subject headings, so the right genre helps the right readers find you.
Finally, genre shapes the writing itself. Pacing, structure, length, tone and even the cover all follow reader expectations. Knowing the rules does not mean following them blindly. It means breaking them on purpose.
Under each genre you will see subgenres, such as cozy mystery or romantic suspense, which narrow the target further. Your book can sit in more than one of them, and a good team plans for that from the first page.
Readers also tend to be loyal. Someone who loves one thriller will buy the next, so a clear genre makes your second book easier to sell than your first.