Some times you will hear that people make more than one backup copy why?
This is based on this simple risk analysis: At any time there is a certain risk that any copy will fail. If you have 2 copies and one fails, then you only have 1. If that fails before you can make a new 2nd copy, you will loose all of your data.
If your are on holiday or it is your backup that fails it can take quite a while before you notice it, and thus the other copy have much more time to fail. The longer time something has to fail in, the higher is the risk that it will fail.
A 3rd at another location will also protect against theft, fire, flooding and other major events.