If you want to use dollars in a smart contract or otherwise in the crypto "ecosystem" (holding them in a wallet), they are useful. If I were Goldman Sachs, I would have made the "GS Cryptodollar" by now, but we're stuck with weird and untrustworthy stablecoins like Tether and USDC.
By the way, the concept of a "stablecoin" (defined as a currency located in a separate financial system than its home) isn't new - you can buy eurodollar futures which are futures on dollars that are physically located in Europe. Ironically, you can also buy euroeuro futures contracts which are for euros located in the US: the euro- prefix on a currency just means "not delivered inside sovereign borders, although the European eurodollar is by far the most important of these.
By the way, the concept of a "stablecoin" (defined as a currency located in a separate financial system than its home) isn't new - you can buy eurodollar futures which are futures on dollars that are physically located in Europe. Ironically, you can also buy euroeuro futures contracts which are for euros located in the US: the euro- prefix on a currency just means "not delivered inside sovereign borders, although the European eurodollar is by far the most important of these.