In order to maintain the usability of the URL in a heavily AJAX application. For example, with movielandmarks I designed it so that you don't use back/forward

In order to maintain the usability of the URL in a heavily AJAX application. For example, with movielandmarks I designed it so that you don't use back/forward at all. But I wanted to provide a useful URL for bookmarking and sharing with friends. So I used the hash for this because changing the actual URL of the page forces a refresh.