If you're referring to downloaded exe & msi setup files and the like, backing them up to an external drive would mean you'd be spared the effort of downloading them all over again if you need to rebuild your system (e.g. following a virus attack or some other form of system corruption). Of course, if they get superseded, you'd want the later versions...

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]