When I started posting my reviews to Usenet (a much larger audience), a more prominent purpose became (and this is probably the primary purpose of most reviews)  to help people decide whether they want to read the books or not. In some cases I have assumed that few will read the books I'm reviewing and have written  to inform people. A small number of reviews were written as  part of ongoing debates in various newsgroups and mailing lists.
Once I started getting review copies of books (around 1995) an obvious motivation was  to obtain free books.
And finally, I wouldn't produce reviews if it wasn't for the fact that  I enjoy writing them - though I sometimes forget this with particularly recalcitrant reviews!