Why backing up your system is important?
Backing up your site, your files, your computer, is a lot like brushing your teeth. You know you need to do it, but sometimes you forget. If you forget for too long you will get bad breath, which annoys people, but is not terribly harmful. If you forget completely you will get cavities, or perhaps loose a tooth.
Backing up your site is the same way. If you ignore it your files may simply just "go away" some day.