Why Do Book Reviewers Exist?

I was hanging out over at the poet Iman Mersal’s blog yesterday (hey, Iman!), and got to pondering the perennial question: Why do book reviewers exist? Aren’t they — er, we — just cruel jokesters who couldn’t write our way out of a paper lunch sack, and thus had to make it up to ourselves by pestering and/or fawning over someone who could?