When you are submitting your review, there is a box you can check to indicate that you would like to receive constructive criticism on your review. As noted on the form, checking the box does not guarantee you will receive feedback from site staff or from any other members of the community. It merely lets other people know you are especially interested in hearing what they think of your review.
This system was implemented because some people post reviews and don't stick around to see what anyone has to say. It can be discouraging to leave feedback when you don't know if the author of the review in question will even read it. Also, some people are hesitant to provide feedback if they're not sure how it will be received. Keep in mind as a writer that sometimes, even constructive criticism can sting. People might not like something you thought you did brilliantly, or they might go off on a tangent, sharing their own experiences with the game or others like it. Remember to be gracious, and it's not a bad idea to say "Thank you" if you receive feedback you find particularly helpful. As a side note, any review you contribute to the site may generate feedback from site staff or readers, whether you specifically request it or not.