With KDE, you can route one application to one audio device and another application to another audio device – at the same time.
When would you ever want to do this? When your son is using your laptop to play Flightgear using the USB headset, and you are listening to music while you work on your Bluetooth headset, because your main workstation does not have Bluetooth…
KDE seems to remember these settings between application launches, so this may be useful for pinning certain applications to certain audio devices.