If you are sending USDT via a wallet, you should be asked to pay some bitcoin as miner fee. This is because USDT is a cryptocurrency asset issued on the Bitcoin blockchain via the Omni Layer Protocol(Use OP_RETURN in Bitcoin transaction to store USDT transaction information).
Therefore, an USDT transaction is actually a bitcoin transaction. Consequently, it needs to pay a bit of bitcoin as the miner fee for an USDT transaction. Similarly，ERC20 tokens are stored and sent using ethereum addresses and transactions, and use gas to cover transaction fees.
To put it simply, USDT needs BTC as gas and ERC20 tokens need ETH as gas.