The feature is rolling out to more devices in 'a matter of days' so keep an eye on updates.
Unfortunately, I’m unable to try the feature myself on an iPhone 13 Pro with the latest version of iOS 15, but that’s par for the course with these particular software rollouts. The ability is still finding its way to devices everywhere, so hang tight if you haven’t received the update yet.
There are plenty of reasons to be excited for a feature as seemingly simple as the ability to watch something in a tiny window while doing something else on your smartphone. For one, YouTube spent a long time avoiding being a vessel for music videos until leaning into it entirely with YouTube Music. You might prefer the service over others because there are tracks that you want to listen to that aren’t streaming anywhere else.