Even if you can use snapshots for testing, I agree with Renegade that it is a good idea to keep backup copies of any original "clean" VMs you create. In many ways, a VM is like another computer available to work on when needed, and you really don't want to lose it by accident.
A VM is like a disk image. The difference is that the operating system on it has been modified to require a virtual environment in which to run. In some cases, it is possible to convert an image of a real system to run in a VM, or to convert a VM to run a real system. I have used Paragon's virtual manager (part of their Hard Disk Manager Pro but also available separately), to restore a backup image of an older system to a VMware VM, allowing me to run programs that were installed on it even though I no longer have the original computer.