User: orundarkes

Here's one use

On: Chrome Tests Picture-in-Picture API To Show Floating Video Popups Outside the Browser (bleepingcomputer.com)

I can see this being useful for help videos on some of the web apps we design so that your average user can play a video, slid it out from the content (preferably on a secondary screen) and follow along to the video without having to leave his current session. I can also see this being annoying for pretty much everything else.