Judging by the patent’s drawings, it looks like the secondary screen can be detached and used as a secondary device, for example as a remote control. It could even double as a QWERTY keyboard when you need it to be, for example while typing out documents or emails, or you could even detach it and use it to send text messages or play games while talking on the phone.
We’re not sure if we’ll see the patent ever made a reality, and if Microsoft is anything like Apple, those patents are more about intellectual property rather than actual devices/technology that we will see, although if such a device did come out with the features we mentioned above, we would definitely be more than curious and would welcome the possibilities that such a device could bring.