Why do academics write/publish book reviews for journals? At least in the humanities, it seems to be fairly common for journals to publish book reviews. To be honest, I don't think I fully understand why many academics write them and why many journals publish them. I don't read them very often because I usually don't find them very enlightening. If I want to find out whether a book is worth looking at, it is usually easier to take a look at the table of contents or else to get it from a library and take a look rather than reading the review.

However, I could imagine the following incentives:


The reviewer thinks the book is particularly interesting and should be known to a greater audience, so they want to promote it.
Writing book reviews is considered a service to the community, and authors would also like to get their own books reviewed in a journal later on.
Reviewers influence the community with their evaluation of a book.
Reviewers accrue prestige by publishing book reviews.


Which of these reasons, if any, make academics write book reviews? If it does add to their prestige, how much? Is it, very roughly, possible to say that one research article equals x published book reviews in terms of added prestige (and increased prospects for promotion)?
In my experience, reviews are written at the request of journal editors and are typically not initiated by the person writing them.  My colleagues and I usually write several book reviews every year because our supervisor is on the editorial board of the journal and tells us to write them.  As reviewers, we do receive a free copy of the book, if you consider that an incentive. There is no prestige associated with writing one, and they don't count as publications in our annual performance reviews.