Everything can fail

The most basic principle in making a good backup strategy is: Everything can fail.

Harddrives break down, DVDs are suddenly unreadable and the fancy "in the cloud" online backup company goes out of buisness.

Think this way: At any time there is a probabillity that a given data storage will fail you. When that happens it will take some time before you realise it, it will take some time to get a replacement storage and it will take some time to make a new backup.