It doesn't require a catastrophic siege-like environment; it just requires that electronic payment systems be unusable (e.g., a cyberattack).

Having cash means you'll be one of the people buying food that weekend, and not stuck in line at ATMs or simply unable to purchase food until the card system comes back online.

Maybe you need plywood and tarps and a few people to help. Maybe your workplace was flattened, but there's plenty to do - I'd rather get cash at the end of the day than a bunch of canned goods, unless I'm truly out of food and it's truly scarce.

I can use that cash for other things like fuel, or stuff that was lost because my house was flattened.

But in a scenario where people are desperate enough to riot for food, there will likely be shortages of other stuff, too. I'd assume it's more intuitive and fair to convert fuel-to-dollars and dollars-to-bread than fuel-to-bread, or days-wages-to-soup, which is the whole point of money anyway.