Any of the above answers are fine. You may have your own reasons and they are equally valid. Once you know why you want to write book reviews, you are well on your way to success. If you look after your readers' interests, they have a reason to trust you and to keep returning to you for advice. If you don't have their interests at heart, your readers will desert you.
Some reviewers simply copy publicity material provided by the publishers or the author.
That is not really a review at all. Pathetic reviewers even copy whole sentences from the front or the back of the book or from pages in between and they don't even give proper credit. Readers can even tell from reviews whether a reviewer has really bothered reading the book at all. A good reviewer also bears in mind that when he or she is trusted by readers, hundreds or thousands of them may go out and purchase the book. It is therefore a heavy obligation that professional reviewers always take seriously.