Wow, is that a thing? Reminds me of an episode from Black Mirror.
No more nefarious or creepy than a screen saver.
I was quite annoyed when I saw it for the first time - I had music playing over my Bluetooth headset while cleaning my bathroom, so it was quite inconvenient to tap on the phone when YouTube suddenly paused it to check if I'm still there.
But if it's just for the ad-supported version, it makes sense.