There are a number of good reasons for installing a virtual machine on your computer -- as a way to run software that isn't compatible with your primary operating system, as a sandbox for development, or as a place to test package installations, new distributions, or new server configurations. Setting one up with VirtualBox OSE is quite easy.
GNU/Linux can be scary to a new user. After all, what if you mess up? What if you end up corrupting your hard drive so badly that you need to format it to get rid of GNU/Linux? The solution is to use virtualization technology. A virtual machine creates a virtual hard drive as well as a virtual computer, so you can install and run it from within another operating system. If you want to get rid of the virtualized (also known as the guest) operating system, just delete the virtual hard disk from the real (host) computer’s hard drive.