Amazon advertises and recommends books based on how many reviews they have. Also on how many of those reviewers bought those books on Amazon (verified purchases), how many times a cover has been clicked on, and more. Now reviews can even dictate if they'll carry a book in their brick and mortar stores. But first and foremost, the number of reviews are vital. Until a book has fifty reviews, it doesn't even hit Amazon's radar as being worth recommending or pushing up in marketing. That means they won't recommend it on the section that says, "If you liked this, you'll enjoy this" or possibly even "Customers who bought this, also bought this". And it certainly won't be placed well on their website where it will be seen if it doesn't have enough.
How important is this if the book is good enough to sell itself? Extremely. Between 600,000 and 1,000,000 books are published every year. That is potentially 2,739 books A DAY that your book, or your favorite author's book are competing against. Without help from retailers like Amazon, or a major marketing budget from the publisher (which is as rare and endangered as an albino tiger), it is nearly impossible for a book to stand out from the masses and be successful enough for the publisher to continue publishing that author.
So authors needs your reviews on sites like Amazon and B&N, as well as your support on social media and word of mouth. Readers are vital to the success of a book. So, if you love an author, review their book, tell everyone you know about it. Otherwise, they could get lose in the vast ocean of publishing and never resurface.
