USDT (Tether) is an ERC20 token. This means its transactions occur on the Ethereum blockchain, which requires that the sender pay a fee in ETH to ensure their transaction confirms. Learn more about ETH fees here.
You will need a small amount of ETH in order to Swap your USDT for any other cryptocurrency within the Blockchain.com Wallet, or to send it externally to a third-party exchange or platform. However, you do not need to keep an ETH balance in order to receive or to store USDT in your Wallet. If you don’t have enough ETH to use your USDT, you can Swap for it from another cryptocurrency or receive it from another exchange or platform.