Eh bank notes have watermarks just to make them harder to forge. They also have serial numbers, but as almost no businesses pay attention to them they aren't really used to track transactions. The provenance of a particular bank note isn't that interesting beyond knowing whether it's a forgery.

The problem with these spymarks is that they can be used to include data that's completely invisible to users - even potentially to sophisticated users and the programs that consume the marked files. So while I can make an informed decision whether to share a picture, I may not be informed about any spymarks. Vs a normal watermark that aren't designed to be invisible.