Refining. Writing reviews actually helps me think more critically about and internalize what I've read. I usually have to read something once to get a grasp on the big picture of it, then read it again to jump into some more of the details and application. So writing reviews forces me to take more time and energy reflecting on both of these aspects, which enables me to internalize the material more effectively. I'm sure others out there are very different from me, but I know that I'm a "get your hands dirty" sort of learner and this is what works well for me.
Finally, a very small reason (like 1%) why I review books is to be helpful to others. Whenever I'm reading reviews of books, which I almost always do before reading or purchasing a book, I find that many of them are very poorly written or constructed. I don't mean to imply that I'm better at it, because I do want to improve and become a good writer. I strive to write reviews that others find interesting, easy to read, and helpful in determining if they want to read a particular title.