I can think of two examples:
1. you pick up an incoming call (voip) , you hear the person on the device speakers. you then find you bluetooth headset, so you power it on. it then gets discovered by pulse and you send the audio to your headset. you also tell pulse to use your headset microphone instead of the built in microphone.

2. you come home , and want to send the audio from your headphones , to the stereo system in the living room. when you enter home, you get connected to the home wifi. then the stereo's upnp is discovered and offered as an audio out, you choose it and hear your music on the loudspeakers.