There is some evidence that a timely review placed in a prominent place can work wonders for sales. There is also evidence that sales are largely independent of reviews. Whatever the truth is, authors and publishers both want reviews.
Reviews are hard to come by.
Publishers spend a lot of money sending out review copies. I have seen publishers and authors bow and scrape and grovel and cajole and beg for reviews. To me, this is a humiliating process. And results are not guaranteed. I ve not heard of bribes used to obtain reviews, but it would not surprise me if this were the case.