Efficiency. You can finish payment while the cashier is ringing you up, and then you can bag groceries while the cashier is finishing the transaction and moving

Efficiency. You can finish payment while the cashier is ringing you up, and then you can bag groceries while the cashier is finishing the transaction and moving on to the next customer. If you have to wait for the total before entering your PIN and accepting the transaction, you hold up the line. Some people might want to wait for the number, but why force me to? If the cashier made a mistake, you can say something to the cashier before they confirm. Worst case you can always reverse it.