Once humans formed communities large enough to support a parasitic structure, i.e. those who did not have to work or contribute to survive, there have always been those on the top of the pyramid living off of, and profiting from, those on the bottom. Why did humans ever agree to this system?
If you write Fantasy, Science Fiction, or Historical fiction, you could explore the thematic question: How does one gain control over the many? Why do people willingly offer up things they have worked for, things that have essential value, to people who offer nothing of essential value in return? Some might say protection was given in return. Since the people form the armies that protect themselves, the argument is questionable at best.
Gold, diamonds, and paper currency only have value because we assign them value. Someone, somewhere along the line, convinced us that this was a good idea. All kinds of nonsense followed.
Why is gold of higher value than bone? Humans tend to value things that are rare, but when the first chunk of gold was found, people didn’t know it was rare, only that it was new. We believe we have found all of the gold, but have we?
Particularly when writing Science Fiction and Fantasy, it is important to decide what your characters value. How do they buy and sell things or trade things. Do they use money as we know it? What are their fiscal rules? What do they value in terms of thrift or extravagance? What happens if their rules are broken? What are the consequences or the cost? Do they have a Wall Street? Do they allow the people at the top to profit from those at the bottom? What is the cost to their society for doing so?
It is even more important in a Historical tale that you get the details right for the time and place. When was the currency put into circulation? What kind was it? If you write about a remote tribe in Borneo, how do they go about bartering? What do they barter? What kind of bartering infractions are there and what is the punishment?
Money is a perceived need. Dick might feel comfortable with a small savings account, or he may not feel comfortable without a very large one. He might cheat, steal, or kill to get what he considers enough. Sometimes there is never enough. Jane might be happy with a little. Put Dick and Jane in a relationship and you have massive conflict.
Some without money resent those who have it. Some with money look down on those who don’t. Some characters work for their money, some inherit it, some win it. Disparities in income cause conflict in schools, social groups, charitable organizations, neighborhoods, families, marriages, between countries, and between friends.
Who should have it? Why should they have it? Should Dick, who spends his days throwing a football, earn more than doctor Jane who spends her days saving lives? Should a pole-dancing Sally earn more than the guy that picks up the trash?
Money trouble is one of the top killers of marriages. It can create an imbalance of power between the one who earns the most or all of the money and the one who isn’t given equal control over it.
When money loans are made, the stakes are raised. You have intense conflict whether you are writing about mob money or the cash dad gave son to help him start a business. The game begins when it can’t be paid back.