Just an avid reader. Mostly SF/Fantasy, some hobbies, paranormal, urban fantasy and lighter, fluffier things.
Genre. G-E-N-R-E.
The book's genre on goodreads is crowdsourced from how book is shelved. I seldom see indie authors on goodreads shelving the books they wrote.
Once in a genre, in addition to sometimes just being plain helpful to readers, your book can display in all kinds of site features and areas (explore by genre, new releases by genre, emails readers subscribe to by genre, generating recommendations, challenges/games linked to genre/shelves which might require reviews, and many more).
If you do shelve appropriately and no "genre" section appears on book page, it takes goodreads 10 minutes to 72+ hours for the cache to catch up to reflect the shelving (and to add the "genre" section to book pages takes two people shelving).
If not sure how to add book to a custom shelf, you can do so under "My Books" or from the drop arrow connected to the "Want to Read" button on book page: