Benjamin Mayo has a great summary of the benefits of Picture in Picture for iOS 9 on the iPad as compared to the Mac:
The thing about the iPad picture-in-picture implementation is that its actually better than how one would handle such a task on a Mac. On a Mac, trying to play a video in the corner whilst getting on with your work is difficult. Let’s take a video on YouTube playing in Safari. To play this in a corner of the screen on a Mac, you have to pull the window out into its own tab. Then, you have to manually drag the corners of the window to resize it and do your best to clip out all the unnecessary surrounding UI by hand. No doubt the window has a toolbar so you’ll probably have to do some awkward keyboard shortcut or hidden menu command to hide that as well.
Then you have to actually manage the window as you go on with your work. What do I mean by this? Well, with every other task you open you also have to make sure it doesn’t occlude the video playback window by dragging it out the way. The video can’t stay foremost so it’s actually really easy to lose the video amongst your other windows.
If you ever want to move the video from one corner to another, not only do you have to position the video on the screen, you also have to move all your other windows back over to the other side.
This mirrors my initial thoughts exactly. When I talk about the inherent complexities of desktop OSes, this is the type of issues I refer to. With an implementation built on years of distilling the experience to the simplest, iPad multitasking will make these differences even more obvious.