Right the nonce–the "dibs code"–is there so that you can't just copy a known sha256 and claim it as your own. You have to at minimum have the file, and to prove you have it one way is to come up with some random data and hash that random data + the actual data. Then others can do the same to show that it indeed matches. Once a nonce is used it can't be re-used for the same reasons.

The AI use case is an interesting one.