File under obvious?
This is likely a desperate attempt to avoid model collapse, where AI feeds itself data and starts hallucinating more and more often. But being able to identify AI data means they can keep AI content out of the data and focus on content created by "the agents that live in reality", aka the entities formerly known as human beings.
Of course that means we also can start filtering AI content out of our feeds and search engines and web sites and training data. That makes watermarked AI output exactly as valuable as it is actually is to society (<0), so I doubt Anthropic would make this mandatory. And if it did make it mandatory, there are plenty of models that aren't doing this that people can use.
This is just the pointless thrashing about of a drown victim, as far as I can tell.