In the devices' hamburger menus, there's a menu item that says "Play all audio via this device". As written, it would appear to do the same thing as clicking on the radio button of a different device in the widget. Or at least, clicking on a radio button for a different device *should* move all audio to that device. And that being the case, we don't need the menu item.
tl;dr This does actually do something different but it's kinda obscure, so who knows if anyone whose itch it would scratch, is actually using Plasma right now. There's a few things going on here that might lead to confusion. The core idea to know is that a client may set target.object stream property ( this is a PW thing but PA does have something similar, which is why plasma-pa has feature in the first place). When set it expresses a direct intention (i.e. user will) for the client to ignore the configured default sink or source. So far simple enough. But here's what can go wrong: 1.The client should get linked to that specified target.object node but certain tools such as EasyEffects always override this and re-link to their input nodes instead. 2. The aforementioned button only relinks streams active at the time rather than enabling some kind of active redirection mode. This is not a bug per-se but it's maybe a bit surprising, since it ignores target.object while also not changing it. Arguably it should change it, since the whole point is user intentionally which was expressed via that GUI button press. 3. This may or may not be a bug in WirePlumber (I think it did come up at some point and is kept if not for any other reason then at least for matching PA behavior) but it ignores clients which set target.object to the default device, so, to actually see the effect working, one needs to set target.object to some 3rd sink (sources work too but sinks are easier to test, IMO), then make a different (sink 2) the default, then switch back to sink 1 and the stream will not have moved anywhere remaining on sink 3. Repeat the process using that "Play all audio via this device" button and the stream will in fact move each time. A 3rd sink is required, because otherwise target.object would be ignored and the client will actually move along either when making sink 2 or sink 1 default due to the WP behavior I mentioned. In conclusion, does it have a niche use? Yes. Does anyone actually use it? I have no idea but presumably someone cared enough to implement it.
I later remembered that this behavior of ignoring target.object when it matches default sink/source was almost certainly introduced, because some clients were misusing the PA API by always setting that value from the default device at the time they were started. Doing essentially undermines default device selection, since clients that are seemingly set to default device would not follow the default change. I suspect that the now obscure "Play all audio via this device" button was introduced as a plasma-pa workaround for the same issue back when pulseaudio daemon had not yet been modified to ignore such client behavior anyway. It's probably best to just disable this in GUI and see if anyone comes complaining about an important feature having gone missing and remove it entirely if no one does cares anymore.
*** This bug has been marked as a duplicate of bug 432487 ***