The USD is the most commonly used currency in the world. The USDT, on the other hand, is a blockchain-based Stable Coin that trades for $1 USD. The USDT is more flexible than USD, but its withdrawal process is slower. For these reasons, some exchanges prefer USDT.
The USD is the flagship currency of global financial markets and sets the direction of global economic policy. In contrast, USDT does not have the same influence on global economies. The USD is backed by the full faith of the United States government, while the USDT is not. As a result, they are not completely equivalent.