AirPlay button in multitasking bar? I don't have an Apple TV, so I've never had an AirPlay button anywhere. However I noticed while switching apps in a call that the AirPlay button appears while making a call. Pressing it does nothing. Also when the call in on speaker the AirPlay button goes blue (and still does nothing on pressing it) Why has it suddenly appeared, whats it's use during a call?

Also I then discovered a program for Mac which allows you to AirPlay to a Mac, increasing the AirPlay functionality vastly. And tryed pressing the button when I had that working, it still did nothing. (though the Mac program works as I can send youtube videos and photos to it)  And in the photos app, opening the multitasking bar and pressing the AirPlay button does nothing. So what is it there for anyway?
It's not just for AirPlay.  It's general-purpose media redirection. You can use it to switch between the built-in earpiece, the speaker on the bottom, Bluetooth earpiece, etc when on a phone call.  For example, I placed a call then went over to the audio controls in the multitasking bar.

It just so happens that the primary use is for AirPlay and media playback.