Question

what is the use of input unit

Answer 1

it is used to provide data and control signals to an information processing system such as a computer or information appliance. Examples of input devices include keyboards, mouse, scanners, digital cameras and joysticks.

Answer 2

Computer is meant for processing information. How do you give information to computer? The way which computer takes informations is through it's input unit.
For example you type information through Keyboard, You paint through mouse, you speak to others using microphone as an input.