There are a few answers to that question:
I read over a 100 books a year and reviews were taking over my writing blog
Friends were asking for good books to read (and my puny brain couldn't remember all the ones I really liked or the reviews)
Finally: It's a great way to practice my writing. On days I can't sit and write in my story, I can sit and write a review.
My blog, The Book Addict, is a place I post reviews for all the books I read. I read about 100 books a year. My reviews have evolved over the few years this blog has been up and running. I found the best way to improve my blog and reviews was to just. keep. writing.