The difference is that your image is married to your hardware. If you decide to buy a new system and retire your old one, odds are that your old Windows image will freak out on the new hardware. Even if you have driver files for the new system, you may not get a chance to install them...
Every time I have attempted to move a Windows hard drive from an old PC into a new PC, the Windows OS wouldn't complete the boot process without throwing up a blue screen.
When Windows is installed into a virtual machine, it marries the virtual machine instead of your hardware. The virtual machine file can be moved from your old PC to your new PC, and Windows will remain happy because it thinks it's still booting from the same hardware.
I wonder if there’s a way to get my current windows setup and settings onto a virtual drive? Probably not, huh?