Every single asset on ethereum need eth if you want to send them. It is for paying gas fees, even you want to send usdt you still need ethereum to send it for gas, and eyhereum gas fees is so high compare to BNB and other blockchain.
USDT has been integrated to a few blockchains: OMNI layer on bitcoin blockchain, ERC-20 on ETH blockchain, TRC-20 on TRX blockchain, EOS, etc.
With the high transaction fees on Bitcoin and Ethereum network, people are keen on using TRC-20 chain to move their Tether USD if possible. If they are moving USDT from exchange A to exchange B, two exchanges must support TRC-20 for Tether USD. Send USDT cross-chain without supports from exchanges will cause loss of USDT you send away.
If you do that mistake, you must contact support and need luck to get your USDT back. You also need to pay fee from exchange if they accept to retrieve your USDT for you.