When you make or receive a call and a Bluetooth headset is linked, the screen lets you route the call to the headset (where it's routed by default) or to the iPhone itself, or to the iPhone's speakerphone. This may seem like a silly little item, but anyone who has dealt with Bluetooth headsets for a while knows it can be a lifesaver: say a call is coming in, and you don't know where your headset is.
