Billions of public Discord messages may be sold through a scraping service

Who would /buy/ this?

Have you heard of any companies in the past few years who are trying to mimic human speech? They need lots of example data to do that.

And what would they do with the knowledge that Fartknocker72 posted sonic slash fanfics?

While knowing that 1 specific person likes something is mostly irrelevant, once you link it to an email or real name (just wait for the next data beach), criminals could use that kinda data for blackmail.

Furthermore, companies like Facebook and Google mostly make their money by linking people's behaviour to their interests. They probably won't be caught with their hand in this cookie jar, but it should show you how valuable this data (in massive quantities) is.