When did it become so important to leave a book review on Amazon?

I see a lot of authors, publishers, readers and reviewers asking people to leave reviews on Amazon. But I wonder why Amazon is considered the top gauging factor in showing support for a book or author? I feel like other review sites should be taken into consideration too.

I often hear that Amazon reviews can make or break a book? But why should one site hold such precedence and when did it become so important? If a book interest someone, a review whether positive or negative is not necessarily going to change their desire in getting it. But the way I hear it, Amazon reviews have become an important step in book selling but why? I don't understand how one site that specializes in selling a multitude of items became so important to the publishing industry.

As a reviewer, I see the importance of writing reviews they help increase promotion and visibility for a book and author. And possibly even increasing book sales and maybe even help a potential reader decide if they would like to read a certain book. A reader may not agree with my opinion but at least with a review they can know more about the book besides what the description states.

I'm not against posting reviews on Amazon (I still do among posting reviews on other sites) but I don't think the reviews on there should matter more than reviews on other sites.