I think the argument is that the buyer wouldn't know whether you still have a copy of the key or not. The idea is that the blockchain is used to ensure you cannot backdate a transaction.
Mind you, I'm not a blockchain fan, and I believe that blockchains and cryptocurrencies are the absolute worst way to solve the problem.
I mean for non NFT art, at a certain point the legal system can prevent you from selling it. Like if you had a piano made of ivory, or if there were international sanctions on the buyer.
Mind you, I'm not a blockchain fan, and I believe that blockchains and cryptocurrencies are the absolute worst way to solve the problem.