The purpose be to claim in court, ownership and/or rights over said works of an individual who has used AI to assist in the production of the output. Even if the actual idea or concept comes from a person, if the AI inserts watermarking into text, the legal fuel is the watermarking of text, which can then be used in court to fight for rights, where otherwise there would not even be any debate.

Imagine a patent drafted by AI under human direction. Regardless of the current laws, this would provide evidence for, and leave the door open for future laws/claims...