A computer program that is designed for data entry including invoice numbers, serial numbers, and package labels can accept data in two ways. The first is with a manual keyboard entry and the second is with a barcode reader. You can always type in data, but this is very slow and tedious, especially if you have to process hundreds or thousands of orders a day.
Barcode readers with a USB keyboard interface can do all the data entry for you. The data is still printed on the paperwork, typically directly under the barcode, in the unlikely event that the barcode reader fails. Or, so that a user can confirm visually that he has the right label without actually having to type it in. Barcode readers eliminate errors commonly associated with normal keyboard entry and speed up the order processing by a factor of 100.