We use the computer in the first place to manipulate data, so obviously you need a way to put the data - in comes the input device. Now the input data has to be processed in the desired format - in comes the Central Processing Unit. Finally you need a means to get back your processed data which is now meaningful information - so we use an output device.
All the devices are related to each other during the entire process of using a computer. Without any one of these we cannot use the computer to its optimum.