Viruses
Viruses are a part of life. Major anti-virus vendors are constantly battling the next wave of malware, Trojan horses, worms, etc. While they do their best, they are constantly one step behind and that means your computer files are at risk—even if you use an anti-virus program.
Should your computer become infected there is no telling how corrupted your files might be. They may not open, function properly, or disappear entirely. While backing up your files online won’t fully thwart viruses it will help you get them back should they become corrupted.