A well written book review can be useful, not only for understanding the book being reviewed, but also for understanding how that book relates to other books on the same or a related topic.
Books reviews also mention the credentials and publishing history of the authors of reviewed works, offering insight into how notable or respected the particular author's work is.
They also provide insight in new ways; for example a historian of philosophy will review a book on 18th century France quite differently than a political historian. This can give you a depth of understanding that might be slightly outside of your current knowledge base.
Finally, book reviews might prevent you from reading a book that was well not reviewed.