Early versions of the Zend Framework had a noRoute action that was called when the correct action couldn't be found. This was a way to deal with some page not found errors. At some point it was dropped - I don't know when or why because I only started using the Zend Framework recently. It's still possible to handle non-existent actions using the __call() method of the controller class. But there's no obvious way to deal with all page not found errors in one place, including instances where the controller doesn't exist.
The framework makes it easy to create actions link to controllers, but there's still a problem when a requested action isn't there. His plugin has a solution to that - it is fired off when the action requested doesn't exist and automatically reroutes it to the "noroute" controller to be handled.