Why, you may well ask? Marketing, naturally. From Nestlé:
"Through the "YouTube my break" branding, Nestlé and Google seek to help people maximize out of their recovery time. The campaign will be based upon the consumer insight that KITKAT® rrndividuals are also YouTube fans, understanding that the video platform is uniquely placed to deliver real time insights to the world's most watched videos."
While it might appear silly, the YouTube Break wrappers are simply part of a comprehensive "break" campaign that Nestlé is running, with 72 others inside pipeline. On Google's end of things, the search giant has set things up so people searching "YouTube my break" after saying the "OK Google" command with their phones will be presented using a playlist of trending videos on YouTube.
If this sounds a little wacky, it really is. However, it's not the first time we've seen unique KitKats because of this partnership. When Android 4.4 was already released, Google and Nestlé ran the same campaign that gave buyers the possibility at winning Nexus 7s and Play Store gift cards using the purchase specially-marked KitKat bars.