Mark is spot on.
Are you thinking that when you change something, it automatically gets backed up to your drive? I doubt your home system is set up that way. A backup is simply a copy of the file at a specific time. If you delete your version, can't access it or accidentally save rubbish over it, you go to your backup and restore it from that date.
It's like taking a photocopy of a photograph. If you then draw a moustache on the original, the copy won't get the moustache as well.