The traditional onsen tamago, not widely consumed outside of Japan, follows a simple recipe of blasting an egg in its shell with hot steam or letting it slow-cook in hot water. It’s basically a poached egg that’s shelled after the poaching.
Seeing as perfectly poaching an egg the traditional western way is a messy and surprisingly complicated affair, it makes a lot of sense to just leave the eggs in their shells and stick them in some hot water for a while, or better yet, just leave them in a hot car, forget about them entirely, and go play some XBox. Next time you go on a beer run, you’ll find delicious onsen tamago waiting for you on your dashboard, like this Japanese Twitter user.