Placing finger twice on Finger Scanner on iPhone 6 plus changes view

I was fooling around with iPhone 6 plus and found that when you place finger twice on finger scanner or rotate your finger on the scanner the currently running app or the menu views move down to half for the screen.

I'm not worried of this behavior but I am curious why such thing is happening.

This is called "Reachability" and is Apple's way accommodating allow for one handed use where the thumb can reach the top of the screen.

It exists on the iPhone 6 line where few people have hands large enough to reach the top of the screen when holding the device from the bottom. Smaller devices such as iPhone 5s and older do not have this toggle. iPads also lack this control.

It is described here: https://www.apple.com/iphone-6/design/