Mainly because of bluetooth headphones with multiple computers. That way they are paired to only one computer and I can use them with other computers at the same time. Just right click on paprefs system tray icon, change the sink and the audio is sent somewhere else. I know it's now possible to have bluetooth headphones that have multiple connections but it wasn't the case a few years ago and I still find it much more useful this way.
But it's also useful when I have my laptop near my main computer and want to use its much better speakers instead of the crappy ones on the laptop. Right click, select another sink, and that's it.
It's just nice to have the option to send the audio from one computer to another. It's a shame that it's apparently a niche thing.