Can anyone explain to me why you need ethereum in your trust wallet especially when you want to send usdt tether Ethereum to another address
You need to understand that Tether is a Token, and Ethereum is the Coin and the Blockchain. Tether is built on top of Ethereum Blockchain, so whenever you want to send Tether or any other Token on a EVM blockchain like Ethereum, BNB. or other blockchahins like Solana, Tron etc using a Non Cusdotial Wallet Like Metamask, Legder or whatever, You would need a small amount of that blockchain's native Coin to cover Gas (Transaction) Fee.
This will change when withdrawing funds from Centralized Exchanges because exchanges take your tokens as fee but from back end they also pay the native token for transactions.
I hope this explanation helps you understand how Ethereum Transactions works now.