The answer is: "Valuable, easily divisible and durable"
Money made out of metals and paper is valuable and durable because of several reasons but I want to mention only two,
If the Joads knew how horrible life would be in California, would they still go? The angry man at the campsite demonstrates how determined families are to find a new life. Even when presented with gruesome tales, they have no choice but to continue on their journey westward. There is no alternative.