The Nexus 6P's big glass window serves as a home for the rear camera, dual LED flash, laser autofocus module, NFC coil (yes, really), as well as an outlet for Wi-Fi, Bluetooth, GPS, and some mobile data connectivity. There is an additional plastic window along the bottom of the phone for the main cellular antenna, as well.
We got this information directly from Google, from a person working on the design of both Nexus devices, so it's legit. And that, ladies and gentlemen, is the story of the Nexus 6P's big, black bar (which is made of Gorilla Glass 4, by the way).