NavRegisterNavFilter is useful if you wish to provide functionality relating to nodes in the navigation tree that you don't own yourself.
The Feeback module in WinGate that uses NavRegisterNavFilter to offer feedback options to the user no matter where they are in the navigation tree.
Another simple example might be one where you write a component that uses the API from the Data UI component. The first time the user navigates to the Data node in the navigation tree you might wish to pop a dialog that informs them of your new feature.
NavFilterNavigationEventFunc has the following signature