The reason to have cash on hand in this digital world is because telecommunications can fail. There can be power outages, especially with natural disasters that cut the power due to wind, ice, etc. If you have "cash," you can still purchase items -- that is if the cashier knows how to make change without the machine doing it for them!
How much cash you need is an individual choice -- and, based on if you have money in the bank. Many Americans do not have any rainy-day savings.
Here is the article on keeping your money in the bank, Do NOT take a bunch of cash out of the bank because of coronavirus.
Last, the No. 1 mission of banks in a disaster is to be sure to have cash available to their customers.