Seriously, for what it would be useful?
Multiple streaming views. Streaming content (Twitch, podcasts, sports, camgirls, e.t.c.) has unique characteristics: It's not edited nor produced, it's real time, and as a result, most of it is boring and not worth the attention.
It may have interesting spots here and there. So it makes sense to watch multiple streamings at the same time from various sources, while doing something else with your time, and when you notice something interesting happening in one of them, you can zoom in and switch your attention at any time. That's where multiple PiP windows would make life 100 times easier.