If society collapsed, resources required to survive have primary value. Food, water, clothes.

But the idea of money will still exist. Precious and rare metals will be worth something in a barter economy.

If you think it would be difficult to defend, you know it would still have value.

The easiest way to defend it is to keep it secret.

This is the way the world worked for a long time. That's why the idea of a treasure map exists.