Cryptocurrencies like Bitcoin don't have that feature, but that just makes them more like fiat currencies. You could argue that fiat currencies are backed by the force the state is willing to use to get its population to pay taxes in the currency, but that's not a stable backing, as the state could decide to accept another currency at any time. Some states do allow for tax payments in other currencies, which means that some fiat currencies have no backing by that mechanism. Some people are also forced to pay in Bitcoin, for example to ransomware, which should have a similar effect in forcing people to use the currency.
Ultimately, I don't see why a currency would need backing. There is nothing wrong with a currency gaining value purely from its value and utility as a currency. Many other valuable things such as precious metals and art gain a large part of its value in the same way.