It's a way of storing documents without needing to use your own computer (or USB storage device, memory card, etc). There's nothing wrong with it in principle, as it's quite secure -- and also protects vital documents against the tragedy of losing your own computer's data. That's a risk if, for example, your computer was stolen, or seriously damaged, and so on.
Bottom line, though, is that unless you have seriously large numbers of huge files that you want to protect, there's no reason to buy any extra space.