While not a traditional customer sale, getting your book into the hands of many readers means your book is talked about more, and a recommendation from a friend has the most influence on readership
among mystery readers.
Not all fictional works, but especially novels where there is an audience that can discover it (zombies, romance,
mystery — sure, some genres have more competition
among books, but this is compensated for by having more
readers), and where the book is pretty good.