Because you want to operate in a way that doesn't involve real currency, either on an exchange without any traditional banking arrangements, or perhaps one that

Because you want to operate in a way that doesn't involve real currency, either on an exchange without any traditional banking arrangements, or perhaps one that doesn't wish to be regulated as dealing with real currency. You may wish to avoid KYC/AML rules, you may want to avoid potential taxes incurred when changing to and from real currency, the exchange may wish to avoid reporting requirements too. You may wish to transfer large-ish sums around between exchanges without any traditional authorities being involved, and in a way that doesn't expose you to volatility. More cynically, exchanges may or may not be able to get these stable-currency tokens (and this is more of a tether concern than UST) at a significant discount compared to real currency, or the exchange may wish to invent currency-like instruments that can be used on its platform without the need to hold real currency reserves.