I imagine there's quite a few benefits, and the odd kudos to be had!
1/ Who could resist the free book to be send you by the publisher? Usually a hardback!
2/ Who might resist the opportunity to critically assess in print the work of a (big name) fellow academic? It's like hunting the elusive Loch Ness Monster! And photoing it!
3/ Don't even try try to resist the kudos of colleagues should you manage to nail a high quality critical assessment!
4/ Never dare poo-poo the Researchgate reads! My best read "Contribution" by a long, long is book review! And a second does me pretty well, thank you!
Apart from the obvious free book to read (usually!) there are some other aspects. Truth be told: there is no money involved, so what else is there? Reviewing books helps you boost your list of publication. If you do not list each and every review you did, you at least note somewhere: "Collaborator of German History, Kritische Justiz, ..." and so forth. That does look pretty impressive on your CV. After having done a review for one magazine, others will ring you up and even though you might start with doing a review for the dissertation of a fairly unknown PhD student, after a while you will eventually get offers for books of Norman Davies and the likes. In the end you might get a reputation for a certain type of books and topics (in my case it turned out judicial history for whatever reason). It does give you an idea how people see you.
As compared to essays on your research topics, reviews are considerably quickly done. You increase your visability in the field. Writing on your current research topics takes time, you might not be ready for it before the end of the research and review-process taktes time. (Sometimes up to two years.) Publishing in a festschrift might get you an acceptance more easy and quick, but the production might also take some time (Three years is not so rare.)
By publishing reviews, you already got accepted. You publish in forseeable time and in a journal at that. You will also establish yourself in the field. Even though you might not be the author of a brilliant book, those who want to get some basic informations about such a book, will eventually check your review to decide whether to read it or not. That way people who never heard of you might stumble about your name and check your publications too.
There is also the benefit that accepting a review forces you to thoroughly read a certain book and work through its topic even though you might not have scheduled it. Well, unless you already had it on your list, but how often have you considered to read a certain book or a book on a certain topic if only you had the time, but your current projects keep you from doing it? Accepting a review forces you to actually do it. You have to make time!
Of course there are people who just scim through a book, read introduction and conclusion and then put together a quick review, missing a lot of content along the way. But you would not do that would you?
Oh, and forgot to mention that you might be lucky enough to have your review quoted on the back of the second edition of the book itself! It's an interesting experience to see how the publisher carefully edits your words to turn your negative view into a positive!!! It happened to me once. I don't remember the details. But it works like this:
You: "This might well be the worst book ever written on this subject. Compared with the one by Dr X, which I consider the very best contribution in this particular area of research..."
Them: "This might well be the.. the very best contribution in this particular area of research...."
Reviewing improves your writing skill and it also helps to hone your skills as a word builder. Moreover you will also become familiar with publishers and the type of books they publish.
Book reviewing, if you do it right, forces you to think a lot more about books than you normally would. It forces you to look at the quality of world-building, writing style, plot, dialogue, characters and so much more.
Book reviewing helps us to understand the differences in chapterisation approaches applicable to various subjects. It provides us deeper insights about a subject area. It would also help us appreciate the existing theoretical frameworks in a research area. It would also expose us to various styles of language. Regards
Keeping up to date with new developments in your field. Having an opportunity to demonstrate your expertise in your field. Contributing to your profession. Many publishers also provide incentives to reviewers, such as payments or discounts on books.
By reviewing a book, reviewer get connected with other authors who in the future might help him. Authors are very thankful to reviewers for taking the time to review their books. Moreover, a good reviewer develops an identity as an expert.