Z-Wave Handheld remotes can typically be used even if the main controller is down. These handheld devices can talk directly to the lights and thermostats, without going through the main controller. They can be a pain to program and keep updates, but these handheld and desktop remotes can be a handy way of controlling lights, heating/cooling and scenes. Handheld remotes such as the Aeon Keyfob, Aeon Minimote can be quite handy due to their small size. You can find handheld remotes by Intermatic, GE/Jasco, Aeon labs, and some others, but for the most part people use their smartphones or scene controllers.
Secondary controllers can also be other Z-Wave controllers in the network, acting as a backup or extention of the network. They can also be handy when upgrading controllers, where the secondary controller inherits the network, then with controller shift, becomes the primary controller.