When a payment app tells you a transfer is complete, you start making decisions. You hand over whatever you've sold, spend the money you've received, or close the app and get on with your day. That little confirmation you get is enough for you to stop thinking about whether the transaction worked.
Source: [CryptoSlate](https://cryptoslate.com/ethereum-isnt-instant-but-collateral-could-make-it-feel-that-way/)