Once you've scanned all your old family photos and historical documents, what will you do with them? You could just store them on your computer's hard drive and hope it never fails. But that would be a very foolish thing to do. All hard drives eventually fail, even if it takes a long time to do so.
Although redundant local backups are one way to go, more likely you'd like to share many of those documents with near and distant cousins. For that, an online photo repository is ideal.