AirPlay button in multitasking bar?
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.