There are other reasons, but shutting down isn't the easiest, nor the fastest, way to prevent others from seeing what's being done on the computer. It may be convenient to have a quick and easy way to shut down one's laptop when finished using it, whether or not anyone else is nearby. For me, one key to open my dropdown terminal and quickly typing 'sudo shutdown now' is the quickest and easiest, but I'm not everyone. With KDE, opening the main menu gives quick access to a shutdown link. I suspect there is something similar in most other desktop environments.