• prevent the user from accidental double-submits, when hitting the browser's reload button
• prevent the user from entering a sequence of actions in the middle, while you expect him to execute them in a pre-defined order (e. g. because you need to rely on session data that has been stored in previous actions)
• support the implementation of generic action sequences that can be reused in various contexts (e. g. a confirmation dialog that can be used when the user is about to delete something as well as when he requests some other irreversible action)
• cleaning up session attributes (e. g. removing session scope forms), when the user finishes or breaks out of a sequence of actions
• prevent the user from breaking out of a sequence of actions you don't want him to (e. g. if you want him to answer a confirmation dialog with either "yes" or "no" and not allow him to leave it in a different way)