You would import the private key with a tool like
Pywallet, or with your Bitcoin client patched with sipa's
key import/export patch.
It's useful mainly for so called "paper wallets", you can save the bitaddress source code to a thumb drive, load it up on a computer never connected to the internet, print the address and private key, and send your Bitcoin savings to that address.
Ten years later, you'd be able to import what would then be either a couple of million-dollar-sport-cars, or just some stupid cryptocurrency not worth the paper the private key is printed on.