I know there are ways to clear these hidden areas on hard disks---but why? Under normal circumstances, no user data is written to these areas; most of the content is system/hardware-related information placed by manufacturers, correct?
For normal use, this has to be correct. An ordinary user who acquires a new, or newly cleared, machine, needs only to be concerned with those areas of the drive that are accessed by the file system. Of course, those engaged in true computer forensic work do have some (well documented) concerns about hidden areas of a drive that can be accessed by special tools, but you are right in thinking that these areas would not contain user data. I could imagine, however, that they might under certain circumstances be used to hide malware or for other nefarious purposes.