A long long time ago, in a far away land, someone spread the word that 50 is the magic number of reviews needed for a book to be promoted by Amazon, on its newsletter, and on its recommendations. More eyeballs on the book, increased possibility of sales. We all want sales, and our fervent desire to gather 50 reviews has caused a reviewing industry to thrive.
Some sell reviews for as low as $5, some as high as $60 (although this high end reviewer makes it appear that you're simply paying to expedite the review). Indie authors, and some unscrupulous business people have used this method to give credibility to the book and attract buyers.