From this, we may know that it is a powerful and useful tool, for those trying to test new Operating Systems, or try new configurations on Known Operating Systems. I use it a lot to test new Linux distribution, without the need to install them on my Hard Disk.
Another great use for it, is to have separated servers on the same hardware just to improve security, I mean: Why to have DNS server, FTP server, Web server and all others on the same environment, where an attack to one of these server may compromise all the others, you can install Linux on a good Hardware, and VirtualBox on it, and three or four different virtual machines running Linux to have all those functions mentioned before on different Virtual Machines, thus increasing the security of your configuration.