A computer requires inputs so it can sense what’s going on in the environment.
A computer requires outputs so it can control the objects around it.
Computer input/output is the basis of all automation.

In the past electronic systems were hard wired so that a button was electronically connected to the bell, so that when a button was pressed the bell sounded. Today the button is connected to an input on the computer and the bell is connected to an output from the computer. The computer can sound the bell when the button is pressed but it can also sound the bell under other conditions e.g. the receipt of a message or according to the time of day.

In the computer scenario there is no direct connection between the button and bell, instead it is virtualised inside the computer. This allows for the creation of many interesting scenarios.