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.

To save some state of your JS application which can be saved to bookmarks (or copy-pasted to someone else). For example, Gmail adds "#sent/12aa04feebaac50e" when I open an e-mail I just sent. Also, to get the "back" button to work properly in JS applications.