You hold USDT on TRON and need it on Ethereum: a DeFi protocol that only accepts ERC-20, a lending or trading venue with an ERC-20 deposit page, a counterparty whose wallet is on Ethereum and who will not touch TRON. Ethereum is the more expensive network to move on, so this is usually done once, for a purpose, rather than for everyday payments.
A TRC-20 transfer cannot reach a 0x… address. The USDT has to be received on TRON and paid out as a separate ERC-20 transfer on Ethereum. That second transfer costs gas; the estimate you see already accounts for it, so the ERC-20 amount is a little below the TRC-20 amount you send.