Debugging and diagnostics. Useful for checking setup, for example.
Lists service class names, as picked up from configuration. Useful with AliasServices (see Section A.2.3, “AliasServices”).
Dumps the contents of the object store. Useful for debugging setup (through SetupObjects, Section A.2.4, “SetUpObjects”, and UsingNakedObjectsViewerForSetup, Section A.3.1, “UsingNakedObjectsViewer / UsingNakedObjectsViewerForSetup”).
Verifies that listed NakedObjectSpecifications have been loaded into the metamodel.
Runs up the DnD viewer with the current state of the objects. This is a great way to inspect the state of the system, for example if a test is failing and you can't see why.
Of course, other alternatives are writing unit tests and debugging with break points. But failing scenario tests usually arise because of an integration issue between two different bits of the system. Being able to have a "poke around" can be invaluable.