Centralized databases are highly susceptible to attack. Over and over again we have seen massive stores of sensitive data hacked, stolen, leaked and tampered with. It only takes one careless person to compromise the whole system. The owners of those databases get a slap on the wrist; the users whose data was leaked spend the rest of their lives in fear of having their money or identity stolen.
By employing asymmetric encryption via public and private keys, blockchains make it possible to conduct secure transactions without having to collect and store personal data in centralized databases. This is good for both the service provider and the user: private keys are more difficult to steal than physical cards, cannot be forged like an ID or written signature, and cannot be stolen en masse like account numbers or passwords since they are not stored in any database. So users don’t get their identities stolen due to lax security systems, and service providers don’t find themselves liable for fraudulent transactions.