One copy is not a backup, and two is barely enough

Copies protect you only when they are in genuinely different places and situations.

A backup is not a state of mind, it is a second existence for your things. If everything you care about lives in one place, you do not have a backup, you have an original and a hope. And the risks are not only dramatic ones. Machines fail, certainly, but so do households: things get dropped, spilled on, taken, lent out, or simply deleted by someone who was tidying up and meant well. A copy sitting on the same machine survives none of those.

What makes copies genuinely useful is separation. Two copies in the same bag are exposed to the same bag going missing. Two copies in the same house are exposed to the same house. The point of keeping more than one, and of keeping them in different circumstances, is that no single unfortunate event can reach all of them at once. That is the whole idea, and it does not require any technical understanding to apply.

The second thing worth knowing is that copies rot from neglect. A copy made three years ago is a snapshot of a life you no longer have, and it will not contain anything that has mattered since. Plenty of people discover at the worst possible moment that their backup is genuinely old. The question is not whether you have ever made a copy. It is when the most recent one was made, and whether anything since then would hurt to lose.