I don't really see this being for drivers, but rather passengers that are using the phone while navigation is active. This brings the instructions while someone else is using the phone for another purpose. The driver should not be touching the phone, so it just stays in the navigation app, full-screen. Is there something that I'm missing?
Other purposes could be when a call comes in or during non-driving navigation (walking, bus, etc.), allowing the navigation to co-exist with other apps that are being used.