Comment by LorenPechtel

This is too strippable to be a good watermark, it would only catch the ones who are unaware. The leakers, yes, the cybersecurity people, no.

Rather, I see a use in signing things. Newspapers, politicians etc, generate a unique key and encode it into your article or whatever. Now it's easy for anyone to check if a quote attributed to you actually came from you. Sure, it's not secure but it doesn't need to be because it's simply a stable identifier. Even paywalled sites could display a snippet around the provided quote without being problematic.