In Jellybeans, what is the long press of the home button for ?

I've had a Galaxy Nexus for a few months now and something I have not been able to figure out still is: when you long press the home button, a semi circle with a white Google Now rounded button appears; well, how is that any useful? You can access Google Now by tapping on the search bar at the top of the home screen as well.. I just don't get it ! Is there something I don't know?