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.
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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.