you can use it to run operating systems parallel while your existing OS is running
e.g. your system is Windows, then you can start linux in just a window (you do not need to restart windows or something like that)
or you can start windows 2000 under windows XP SP2
also good thing: you can save the image and give this to the developers and they have the possibility to see the failure her-/himself.
of course there are some other nice side-effects like: you have a clean windows and you can test installers very easy + fast (otherwise you need to use Symantec's Norton Ghost or Acronis's True Image,...).
