If something were to happen to your computer, you could lose all your data!
Having a second place to store your data is a very good idea.
A lot of people buy an external hard drive, which you can connect to your computer and make a copy of your data, so you can be assured that you have another copy, just in case the worst happens.
Another way of backing up your data to store it "in the Cloud" using an Internet Backup Service.
Remember to backup your data regularly and keep the copy in a safe place.
Another way of backing up (or archiving) data is to burn it on to a DVD (or CD).