Outter screen would be low power to save battery life
We've seen mobile phones with two screens on them many times over the years. Typically, these phones are clamshells with a little display on the outside that shows the time and other indicators. Microsoft has a patent application that turned up showing a display on the outside of the standard tablet form factor smart phone.
The idea is that the lower power display on the back could show things such as the time, custom tattoos, and indicators for messages and more. That low-power screen would help save battery life by not having to turn the higher power screen on. The patent app also goes over a way that the external screen could be used as a countdown timer for self-portraits.