Gambling! That's actually the number one use case by far. Far beyond eg buying illicit substances. Regular money is much better for that.

Matt Levine (of Money Stuff fame) came up with another use case in a corporate setting: in many companies, especially banks, their systems are fragmented and full of technical debt. As a CEO it's hard to get workers and shareholders excited about a database cleanup. But for a time, it was easy to get people fired up about blockchain. Well, and the first thing you have to do before you can put all your data on the blockchain, is get all your data into common formats.

Thus the exciting but useless blockchain can provide motivational cover for the useful but dull sounding database cleanup.

(Feel free to be as cynical as you want to be about this.)