When you buy a computer, what you're getting is a case with the necessary hardware to make the computer run. The bare minimum you'll need is the computer itself, a monitor and a keyboard. Although most modern operating systems are designed to be used via a graphical user interface and a mouse, even a mouse is an optional component. Everything else you buy (such as a printer or scanner) is an accessory, and whilst not strictly necessary to the operation of your computer, can make using it more enjoyable, or more productive.