Why do I say this? Well, for two reasons.
Firstly (not in order of priority) they bring stories to the attention of those who might not otherwise find them. An example is one I've just added to my favourites - "Finding an Editor" by qhml1. There are just so many stories and so many authors on SoL it's much too easy to miss good ones. Perhaps I ought to be more adventurous, or something. But seriously, this is a good one; read it.
Secondly, as an author I love feedback - who doesn't? But when the feedback drops off I derive enormous encouragement from reviews of my own work.
So here's a rousing cheer for our reviewers. Friends, I thank you for your efforts. They are valued and valuable. I know how folks look down on critics, at least the ones who don't write themselves, but don't be put off. Please.