why did society collapse? It is hard to come up with something realistic. There are a lot of shocks that will make things bad for a few weeks, but society will recover - or at least the survivors will.

I can think of two, but perhaps you can think of more.

First is the local society collapse because of war. Could happen to everyone, and while your armies might win in the long run, you might be forced to flee. At that point gold is useful because it is small enough to hide on your person, thus meaning you have a chance to get it out to a safer area. You might not be able to prove you own foreign bonds (or maybe you can, but it takes years of paperwork). Gold still has value to the rest of the world in this, so if you can get out with it that is a good thing.

A nuclear nation decides to end it all and shoot randomly targeted ICBMs everywhere. In this case the few percent of the world that survives by luck will need to start over. It is just your village, you can't travel far because of the wastelands surrounding you. Gold is useful because it can be formed into tools. Iron is better, but harder to form, and you may not have fuel to spare to heat it (proper heat treatment of steel is one of the things that makes iron useful). Gold also is pretty and so there will still be the jewelry aspects.

Neither of the above require the gold be currency, though it is a good choice for currency in general in the latter case when starting over. (not the only good choice) Scarcity is part of what has always made gold useful. You could get it in enough quantities that most travelers could carry their wealth around in that form (when not in the form of trade goods - traveler implies trader in those historical days)

Both of the above are long shots. I don't personally invest in gold because I find the risks of the above low enough that I don't bother to insurance against them.