We might judge a very good author more harshly than someone who is newer and just stretching their quill. If we leave harsh concrit for a newer author's story, without her even being able to identify herself and enter a dialogue, it's not a good way to gently encourage new authors.

*Devil's Advocate mode on* On the other hand, as coffeejunkii said, it might be a way for "big name" writers to get an honest opinion on their fics that they may otherwise not get because of the "Who am I to criticise that fabulous, fandom-famous writer?" factor. (Yes, the alliteration was deliberate.)