Netflix shows a module to signed in users at the top of their home page asking the user to rate recent returns. The module promises to give 2 recommendations immediately after they've rated. On sites like Flickr, the recommendation engine is perhaps one of the single best incentives for adding ratings. On a lot of sites, ratings give you nothing back. This is a good example to illustrate quickly why you'd want to add a rating, and would be particularly useful to demonstrate the value to new users.