With this feature enabled, you can use left or right head movements to act like you're pressing on the home button, to open the notification center or app switcher, start Siri, raise or lower volume or to count as a tap on the glass.
Surely, Apple added this to iOS to combat some of the alternative gesture based navigation available on the Samsung Galaxy S4. The latter can turn pages with a gesture to the left or right and Air View allows the user to hover his finger over the glass to read emails and view pictures. The problem is that these are not seen as being innovations, but are considered gimmicks since most people are not going to turn their head to the left or right to navigate on the Apple iPhone, at least not in public
source: 9to5Mac via Gizmodo