Simultaneous Peripheral Operation On-Line
In the early days of computing (early 1960s), before multitasking was invented, computers (e.g. IBM 704) could run only one job at a time. As peripheral devices such as printers or card readers were much slower than the CPU, devoting the computer (the only computer in many cases) to controlling such devices was impractical.
To free the CPU for useful work, the output was sent to a magnetic tape drive, which was much faster than a printer and much cheaper than a computer. After the job was finished the tape was removed from the tape drive attached to the computer and mounted on a tape drive connected to a printer (such as the IBM 1403). The printer could then print the data without holding up the computer. Similarly, instead of inputting the program from the card reader it was first copied to a tape and the tape was read by the computer.