The primary purpose of private currency (like stablecoins, bank deposits, or banknotes) is to alleviate shortages of publicly supply currency. That is, to provide liquidity to meet the demand for money within a locality or venue. It’s difficult to manufacture an unmet need for currency, which is why so many stablecoins today resort to providing yield, but historically, it is providing lubricant to trade and transactions that make for an enduring demand for private currency - whether a bank balance tied to your debit card or small denominations in short supply or being safer to store/transport.