You hold ZEC and need dollars where Ethereum apps, lending markets and most large exchanges expect them: USDT as an ERC-20 token at a 0x… address. Ethereum is the dearest of the USDT networks to transact on, so this route suits an amount that will sit in the wallet or go to a service that credits ERC-20 deposits.
A Zcash wallet sends only to Zcash addresses, and Ethereum reads nothing from the Zcash chain. The ZEC is received here and confirmed on Zcash — 6 confirmations at the 75-second block target, about 9–19 minutes — then USDT is transferred on Ethereum to the address you gave. Gas for that transfer is paid by the sending address, so the payout needs no ETH from you.