Virtualbox is a software that emulates the hardware of a computer within your mac. You can install a linux or windows operating system on it. I chose to install linux because it is free and can be very minimalist and use a few amount of RAM.
I need to access 10.9 from my older machine because I still use it to do some work (drawing work) and the hard drive on that mac is a bit small so I directly work on the mac pro hard drive from the old machine. Now you could ask why still working on an old mac while a mac pro can do everything faster ? Actually, It is not always the case. I have setup a powermac 9500 with a G4 700MHz processor, a lot of RAM and running system 7 on it is very fast. So when I do hand drawn animations, this computer is still the perfect machine to quickly sketch my drawings. I have both computers sitting next to each other on my desk and share the same files on the big hard drive.