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#11
Disk images are a precise copy of one state. Thus if you create an image, and restore it, you get exactly your partition as of the date you imaged it.
Imagine your disk fails. You have to clean install. What a pain!
If you have a disk image, you can install a new disk, and restore your disk image. Job done. Time saved.
That's just one example - all regular contributors here use disk imaging, as far as I know.
And did I mention - an image is also a full backup from which you can extract files and folders?