Let's begin by assuming you have original (CD-ROM or DVD-ROM) copies of your operating system and all installed software. Now consider this question:
If your hard drive suffered a complete failure, how much time could you afford to spend restoring it to working order?
If you use your computer to run a business, do your homework, or trade stocks, for example, your answer may be "a few minutes at the most." If no critical projects depend on a functional computer, you may be able to afford several days to restore it after a failure. Most of us are somewhere in between.
In the best case, it will take you several hoursand possibly a day or moreto reinstall a typical set of software onto a new or reformatted disk. However, if you do not have original copies of all your software, if you have a large number of third-party applications, or if you've customized your computer extensively, returning your computer to operation could take much longer.
The more you need to avoid that potential loss of time, the more you need to maintain duplicates (for more info, see The Duplicate, page 92).