Fundamentally, what a blockchain lets you do is it lets you write code that can make strong commitments about how it will behave in the future. To give you an example, the most obvious one is Bitcoin ... There will only ever be 21 million bitcoins. That property of saying there's only going to be 21 million is guaranteed by the blockchain. It is not guaranteed by the creators of Bitcoin. It's not guaranteed by the developers of Bitcoin, by Satoshi Nakamoto. It's guaranteed by the very network architecture.