Most people read "system" to mean an individual computer, and think that all a sysadmin does is clean viruses off your computer and replace your monitor.
A real computing system is larger. Very few computers work just on their own anymore. When you use the web, play a game online, share files with a friend, or send email, you're using a complex and intricate collection of computers, networks and software that come together to do the job you're asking.