Book reviews are useful for quickly evaluating books you are considering for your research.
They can also help you gage the critical reception of older works when they were initially published and help you get a feel for the scholarship in your field during that time period.
Reading book reviews can help you develop your research skills as well. Learning to read actively and critically evaluate texts will help enormously as you write up your own research and engage in the scholarly conversation in your field.
Reviews in popular sources like newspapers and magazines will appear shortly after a book is published and usually contain a brief summary and critical assessment of the work to help you decide whether or not you want to read it.
Reviews in scholarly publications may appear years after a work is published. They typically offer a thorough analysis of the text -- its thesis, arguments, and sources consulted -- and consider how it compares to other works on the subject.