Cryptocurrencies create value by creating a financial system that solves some problems that traditional finance can't. For instance, a person in some underdeveloped country can accept a payment from US, however, if there was no Bitcoin then the person would have to rely on SWIFT network (which the country might not be a part of, or under sanctions like Iran). Or just a fact that you can't be randomly locked out from your account due to cancelling or any other reason. Buying a portion of Bitcoin is like buying shares of a bank which provides a service of accepting, sending and storing your money.

Ethereum creates value by providing an infra for dApps that provide financial instruments (like deposits, lending, etc) which also create value.