we read books that pertain to our academic interests. oftentimes, books are superior to journal articles since they give accessible overviews of a literature or general area of study whereas articles focus on the minutiae of one particular experiment or inquiry.
writing book reviews is a good way to bulk up your CV since they're relatively low cost, especially if you're going to read the book anyway. all you need is 600-1200 words and one or two insightful comments. ideally, others can read your quick review and judge whether the book is going to be useful to them.
why, then, are there so few journals that accept reviews? i've been searching for over an hour for an appropriate venue for a review, with absolutely no luck. i think the propensity to focus on impact factors makes it a necessity to use valuable page space on items that will be cited. since book reviews are not (cited) then there's no benefit (from an editor's standpoint) of including book reviews.
i would argue that since books are valuable tools for the advancement of knowledge, and since it takes a long time to read one, having concise, well-written reviews can only help the practice (of any discipline). we should therefore have more journal space devoted to book reviews.