The Job of the Book Reviewer

I've been giving this a lot of thought since Monday Night (when I had a chat with the Facebook group 10 Minute Novelists). 

What exactly is the job of the book reviewer?

We reviewers do a service just by publishing book reviews. Reviews benefit the author in a number of ways.

• Reviews on Amazon boost the visibility of an author's work, regardless of whether the rating is high or low.
• Reviewers provide valuable feedback to the author, indicating areas that need improvement or encouraging the author in his or her strong areas.
• Book reviews increase the visibility of books, particularly where those reviews are shared to social media.

They also benefit readers by helping readers to choose books which they are most likely to enjoy reading (and to finish). This also helps authors in a secondary way (by promoting their books).