Human brains are super fast, but te input and output devices of the computer called the brain are real bottle necks. They can do only so much at a given time, and so to increase the speed of a certain process, you will need to employ more people, and that is a waste of money, so the cheapest alternative is automation, or usage of digital computers.
Writing receipts manually listing down all rates, calculating the total and all can be time consuming. Tough job. thats why such tedious things should be replaced with a receipt printer that will take care of everything for you, and you can close a sale in less than 20 seconds while the manual way would take about 5 minutes.