Successful books have lots of positive reviews on Amazon, and it's no coincidence. It's another point in the positive feedback loop: Good books garner good reviews, which leads to more sales. Good reviews on Amazon are particularly crucial for books by new authors and niche books.
Positive reviews on Amazon boost your sales not only on Amazon, but everywhere people are buying books. What percentage of buyers at brick-and-mortar bookstores actually made their choice by reading Amazon customer reviews? There's no way of knowing exactly, but rest assured it's a substantial and growing number.
Amazon's reviews are effective because they're often written by people who are knowledgeable and passionate about the book and its topic. They're seen as an objective evaluation from someone with no ax to grind. Sure, many inept and biased reviews appear, but they're easily ignored and far outweighed by the good ones.
In the case of a niche book, an amateur reviewer with the right expertise in the topic can critique it better than any professional reviewer can.