I don't understand either. It seems the use case is limited to pushing a notification to all devices. I know plenty of apps do that, but it is generally just spammy.

Any app that needs to send custom push notification to individual users needs to have connected the user_id to the device_id. Once you go the Urban Airship route, there is basically no way to do anything useful with push notifications.

Parse seems to be the only 3rd party service that can do something useful, but you would need to use their user/authentication system.