What if this is a fingerprint sensor as well with shortcut assignment capabilities.
Some phones have fingerprint sensors on the back under the camera, others have them on the front at the bottom. Why not have 2 and be able to map certain actions to each?
I'd rather unlock my phone with my thumb on the front button, and use sensor on the back for something else....like taking pictures (especially in portrait mode). Flip the phone to landscape and the assignment changes on the front sensor. Now you can take landscape photos with the sensor on the front, or not.
Edit. Check out the features of Lenovo's Z2 Button on the front....hopefully if there is a button on the front of the Pixel, it can do this stuff.
Consider it similar to the active key on the S6 and S7 Active. Do whatever you want with it to access features or apps important to you. Maybe even you can use gestures with your thumb to launch up to 4 apps/shortcuts.