All your digital photographs, software programs, home videos, music, hardware drivers, documents, etc are copied to another safe location. So even if your main hard-disk runs out of service, you don’t loose any of those irreplaceable treasures.
When you are upgrading the hard-disk of your computer, you can just create a disk image of the original hard-disk and then migrate that disk image onto the new drive. You don’t have to undergo the painful process of reinstalling the OS, all the software or migrating the documents from one hard-disk to another. It’s all done by the disk imaging software.