You hold ETH and need a dollar balance where it is easiest to spend: an invoice in USDT, an exchange deposit page with a T… address, a payment to someone who only uses TRON. Or you want to step out of the ETH price into a stable balance, and TRON's low fees make it the convenient place to park it.
Ethereum cannot send to a T… address. The route receives your ETH at a one-time 0x… address, waits for 15 confirmations — about 4–10 minutes — and transfers USDT on TRON to the address you gave. The USDT amount is fixed at the fifteenth confirmation; until then the estimate follows the ETH price.