Blockchains are computers that can make commitments. Traditional computers are ultimately controlled by people, either directly in the case of personal computers or indirectly through organizations. Blockchains invert this power relationship, putting the code in charge. A game theoretic mechanism — a so-called consensus mechanism — makes blockchains resilient to modifications to their underlying physical components, effectively making them resilient to human intervention.
As a result, a properly designed blockchain provides strong guarantees that the code it runs will continue to operate as designed. For the first time, a computer system can be truly autonomous: self-governed, by its own code, instead of by people. Autonomous computers can be relied on and trusted in ways that human-governed computers can’t.
Computers that make commitments can be useful in finance. The most famous example of this is Bitcoin, which makes various commitments, including that there will never be more than 21 million bitcoins, a commitment that makes bitcoins scarce and therefore capable of being valuable. Without a blockchain, this commitment could have been made by a person or a business, but it is unlikely that other people would have really trusted that commitment, since people and businesses change their minds all the time. Prior to Bitcoin, besides precious metals which are naturally scarce, the only credible commitments to monetary scarcity came from governments.