Why have just one operating system
I'm currently running three different operating systems on my desktop PC. There's my habitual Kubuntu Linux, a version of Android (more common on phones), and Windows. They're running simultaneously and I can select any one with a mouse click.
This magic comes about by the use of "virtualisation" - which simply refers to the creation of a virtual (rather than an actual) version of something. With the right software and a reasonably powerful PC, you can build virtual "compartments" inside your computer that look and act like a whole separate machine. That allows you, for example, to run Linux and Mac on Windows, try out a new web server or run an old operating system such as DOS or OS/2, all without disturbing your current system - or having to reboot. "Guest" systems can communicate with the underlying (or "host" operating system) via shared folders, networking or the clipboard, and you can install and run as many VMs (virtual machines) as you like, with disk space and memory the only real limitation.