Crypto has exactly one such usecase.

Darknet markets. Lots of drugs and lots of illegal materials are bought and sold using crypto, like stolen cards and child porn.

So, “if you are child porn enthusiast, you are maybe using cryptocurrency” might be a valid usecase.

However I don’t know how prevalent crypto is in any of these. I haven’t read any studies on it, and they might be hard to do.

I don’t think any of the big drug cartels actually use cryptocurrency, for example, but I might be wrong.