Why is it important to back up files to a removable storage device?

My last computer was prone to crashing, viruses, etc. Music I had purchased over the internet would be lost, forcing me to re-purchase it, as well as reloading licensed CD's in my possession. Digital photographs were lost that could not be replaced (I lost 2 years of photos).

In short, a back-up drive is like an insurance policy- If your computer dies, or if it is hit by a virus, you simply up-load files. It is a good idea not to leave the back-up drive connected to you computer, to avoid infections.