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Most of us use our computers for online banking, and even if we're careful about telling our browsers never to remember that password there's a million ways it can creep onto the hard drive anyway -- from simply being in the swap file to the poor (but ubiquitous) practice of using the same password for other sites we do let our browsers remember.
There's a wealth of personal data on a hard drive that makes it a potential gold mine for identity thieves. Even if you think you're really careful about that, it doesn't hurt to take that extra step and wipe the hard drive.
With a single write of zeroes everywhere. Because not even the best-equipped labs can figure out what was written there previously on modern hard drives. If you happen to be using a 30-year-old Cray, though, you might need to be a little more careful. But probably not.