Every computer needs an operating system to run other programs.
Operating systems perform basic tasks like receiving input from your keyboard, sending data to the monitor, keeping track of files and directories, and controlling disk drives, printers, and security passwords. It's kind of like a traffic cop that runs in the background when you are using different applications. You seldom think of an operating system, but it's there. This one is familiar to all home users. This happens so quickly, it gives the appearance of performing all of the programs at the same time.