Without the monitor, a person using a computer for their own personal use would not be able to see what the computer is outputting. Whilst in organizations computer might be used and not required to have a monitor, consumer computers require monitors so that users actually know what they're doing on the screen. A computer will be no good or watching movies, making word documents or creating power point presentations when there is no screen. So if we're talking about regular computers that are used at home and in the office, the monitor is obviously one of the most important features of the computer.
Obviously, the main part of the computer system is the actual tower where all of the computer parts are stores. This is like the 'brains' of the computer. It contains all of the computer's memory, as well as its storage space and processors. Without this box then the computer simply wouldn't exist. The processor is required so that the brain can 'think', in one sense, and the memory is required so that an operating system can be run on the machine and that it will be able to display things on the monitor and actually provide some kind of purpose to the user.
Without the keyboard and mouse, the user would not be able to tell the computer what to do. You will be able to see the start up screen and the log in screen, but due to the fact there will be no input then all you will get is a computer that displays the login screen of your computer. The keyboard was once the main way that input was provided for a computer, using codes and script to control the computer. As the operating systems advanced, mice were introduced and now we even use touch screens.