It is useful when you have to watch two or more important live events simulanoulsy like a live event and a twitch stream that you dont want to miss either, it is an essential part of multitasking no restriction should be enforced over media pip, in the contrary it should be assisted and made easy to be reorganized and magnet locked to eachother and to screen border and any other option that makes each stream be fully controllable in this mode like full screen or put it on audio focus if you have too many videos to control etc... heck they should put the whole browser in pip mode so you can browse while watching other media simultanouly, pip mode should be the foundation of the whole browser design, new tab becomes new pip window or open link in pip mode... the best browser is the one that handles media the best and multiple media for that matter, if you have multiple tabs open it is more likely that there are one media in each one of them ready to be played so helping the user organze them in a pip mode would be the best thing he would hope for...