[DennisP]

Fwiw, the second-largest blockchain barely uses any energy. The real problem is that it's still not scalable enough for something like this, but it's improving pretty rapidly.

As for why blockchain: if you did this on a regular database, you'd be trusting a third party to run that database. Businesses tend not to like that, because sooner or later it gets hard to switch away and the third party starts charging a lot more money.