One use case, is clear, independently verifiable ownership of non-tangible things, like Intellectual Property rights.
Why is your system better than the existing one?
There is, for example, the first-owner problem in a public blockchain. What happens if I make an NFT saying I own you property? Without an external system, how can you prove your NFT is real and mine isn’t? And if there’s an external system, why not use that instead?