How to read an entire disc?

There are plenty of other uses as well, like:

1) Data Recovery. Reading an entire disk will allow to to recover (a large part of) deleted files after they are removed from the trashcan, or in the case of a corrupted file allocation table.

2) Disk Cloning. By doing a sector-by-sector reading of a drive, you can make an exact image of it, which you can use for backup/restore purposes.

I'm sure that there's some other uses as well -- but bottom line is that there are plenty of perfectly legit uses for such techniques.