The OP says: What value does gold bullion and jewelry have, when you can barely get enough water and food to survive?
Virtually none. This is the scenario: Your character: A healthy male in his 40s, thin, but you have not had food in two days, and you haven't had water since yesterday. You are exhausted and have nothing stashed. The land is picked over so there is nothing to be found to eat. It hasn't rained in weeks and rainwater is the only thing you trust to be uncontaminated.
You come to a fork in the road, literally, with recruiters offering you two different deals. Presume, for the purpose of focusing this example on the true dilemma, that you can trust both men to keep their word; they will give you what they promise, and you must give them what you promise.
Both men are charged with recruiting a workforce to build, break land, and clear forest for newly formed farming and ranching communities. Neither man will tolerate a changed mind: If you turn them down, you cannot return later to take them up on their offer. The men are friendly with each other, their communities will be sisters. In exchange for an apple from their shared bag, they examine you thoroughly, both wish to make you an offer.
The work is the same hard labor for both: carrying bags of concrete, pulling carts, digging with pick and shovel to clear land, building stone walls. You will work your nearly dead body for 12 hours a day.
On your left is Aaron. He offers you, in return for each hour, all the clean water you can drink, and 500 calories of vegetable and chicken soup (with bread), and 10 minutes to consume them and rest. Every hour, you will work 50 minutes, and for 10 you can eat, rest, eliminate, etc. At night you have 12 hours of shelter (a tent), a community fire, and the companionship of your fellow workers if you want it. Altogether, you get 6000 calories a day, about what you will burn for a hard labor life.
On your right is Bill. He offers you, in return for each 12 hours of work, a gold coin worth, IRL, about \$100. So about \$8.50 an hour, like a low wage job in 2017 America. He will also offer you a tent and community fire, but it is up to you to find and buy your water and food.
Note that IRL, Aaron's deal is not even close to minimum wage. There are 1085 calories in a pound of chicken, which is always available at my grocery store for about $2/lb, so roughly 500 calories per dollar. This means 6000 calories of chicken, IRL, costs about \$12. For 12 hours of work, that is only \$1 per hour.
But this is vegetable chicken soup, and many vegetables cost far less; see This Chart, which is a little out of date but still has the rankings of food by cost per calorie roughly correct.
Many of these foods would not be available in a post-apocalyptic society, but presumably food and grain still grows, and chickens can be fed on peas, corn, beans and insects (e.g. termite farming can provide a good source of protein for chickens, and the chickens will do much of the work once you stir the nest). So let us say that in the end Aaron is offering you the equivalent, IRL, of 42 cents a day, $\frac{1}{20}$ of what Bill is offering you, \$8.50 per day.
Yet most of us take Aaron's deal. There is no guarantee we can buy food or water, Bill is not promising to sell it to us, he offers us only the gold. Our fellow workers, in Bill's camp, don't necessarily have food and water to sell us, and we don't know where we would buy it (except perhaps from Aaron's camp, but workers there don't get to hoard food or water in order to sell it; they consume their pay each hour and that's it).
Here is the major misconception people have about currency, rarity, and "intrinsic value": None of that matters more than trust. If your hungry and thirsty character, the worker above, has no faith that they will be able to trade a gold coin for food and water, then they will not work the first day for a gold coin. A currency does not have to be "rare" to have value, the only demands are that it is difficult to create counterfeit money (a trait that rarity can provide, but see my note on rarity below), and that the holder of the currency is virtually certain they will be able to trade it for goods and services they want: They have faith in the currency.
Note on rarity: The physical rarity of materials (like gold and jewels) is not the only thing that makes an item hard to counterfeit; our own paper money is difficult to counterfeit.
"Difficult to Counterfeit" is really an adjunct to faith and trust in the currency; if it is not difficult to counterfeit, nobody will trust it. If it is subject to wild inflation, or fluctuates too widely in trading value, then nobody will trust it either; meaning they won't believe it is worth, approximately, the same amount of labor it took them to get it.
So in the end, the value of currency is all about whether almost everybody believes it can be exchanged for goods and services. It is not about rarity, or inherent value, or utility. It is about belief.
If nobody in your world believes gold and jewels can be exchanged, quickly and easily, for the food and water in scarce supply, then gold and jewels will not act as currency. They will be worth about what pretty stones and shells are worth. In a post-apocalypse society, the most plausible outcome to me is that tools, weapons, food and sometimes medicine will be far more valuable in trade than gold or jewels.