Book reviews can help a reader, scholar, or even library or university if a book or other piece of media is useful.
For general or popular literature and media reviews can help sway a potential reader to buy a product, or go see a movie.
Critical reviews help determine not only if a book is going to be good, but if it covers a topic well, and can be used to help institutions decide on their purchases.
Scholarly reviews help researchers understand what a book is going to be about without reading the entire work and help institutions, libraries, and other places of learning decide what to purchase. Scholarly reviews can also be an interesting critical work in and of themselves, as they are written by scholarly experts in the field in question and are published in peer-reviewed journals.
When you're doing your research and looking for an article, an abstract can be extremely important. Basically a summary, an abstract will cover the most salient parts of the article in question.
Book reviews can sometimes do the same thing for a larger work. However, keep in mind that this is not always the case. Scholarly reviews often can act as an abstract, and will sometimes go into a chapter-by-chapter analysis, but critical and popular reviews do not always and may not ever do so. Critical and popular reviews, though, can provide cultural or general interest context for a work, and can help situate a book within a particular time and place. If you are researching a new book you may already know this context, but historical reviews for an older work can help fill in details without having to read an entire book!