Tether's point is to be a fast way to get in and out of unstable cryptocurrencies. If you convert BTC to cold hard USD, you have to talk to a bank at some point and that's slow and imposes KYC requirements. Converting BTC to USDT, on the other hand, is effectively instant, and Tether's promise is that one USDT will always be worth one USD.

The catch, of course, is whether that promise is true and if it's even possible to convert your USDT to actual USD, and the answer to both seems to be "no"...