Imagine you have a video on your website and you have a text that goes along with it. By using picture-in-picture you can make sure that THEOplayer remains visible even if the user scrolls down. It's up to you to decide how much of the player has to be hidden between modes.
Now you can always have your visitors in touch with your content, so they will never miss a moment. Maximize the impact of your content so your users get the best out of it.