There are many reasons that reviews, especially early and plentiful ones, can help your book.

• Generate Buzz/Word of Mouth – reviews get people talking about your book, and if people are talking about your book, then other people are hearing about your book, and that leads to…
• Increased Sales – Many people put off purchasing decisions until they hear what others thought about the item in question first. Do you read reviews of movies before you go see them? Look up reviews about a new appliance you’re thinking about buying? It’s the same thing with books. The more reviews you have, the more weight your release seems to have. All else being equal, if the choice is between two books of interest and one has a hundred reviews and the other has only a couple, or even none, most people lean towards buying the book that others have taken a chance on first
• Broader Marketing strategy – There are many ways to generate interest in your book, and the social influence of having reviews is just one of them.  Another way is by using newsletters or other promotional services, but some of them (such as Bookbub) require that your book have a certain number of reviews before they will even consider you