where I live its mandatory for all sales to be registered live with the tax office

My cash worked fine getting some extra groceries at the store when there was this Iberian Peninsula wide (so Portugal + Spain) daylong blackout the other month.

People without cash were screwed. Some were complaining of having no drinking water (because without power the water from the utilities was soon out as they couldn't run their pumps) and not being able to buy any because they had no cash to pay for it.

Also worked fine when we got hit by a freak storm that trashed lots of trees and plenty of roofs and took power down for 4 days, and I'm in a small city where utilities quickly got fixed - some people out there in small villages were still without power almost a month later.

Mind you, people paying by phone would be even worse - most phones run out of power in a day or two unless you have an external power bank to charge the phone (which I do, but most people don't).

None of this event was some giant deadly thing - the first was a loss of control on the Spanish side ofthe power grid that cascaded into a massive blackout as almost all powder generation ended up switched of and had to be brought up slowly block by block whist keeping generation balanced with consumptions and the second was a strong geographically very focused storm effect with high speed wins during the night that brought down power poles, including the high voltage power distribution ones.

There were no floods or more than a handful of deaths, just lots of topple poles and trees and roofs that lost tiles, so there weren't really any much more pressing issues than having no power and hence no water, with the former leading to unecessary extra problems for people who had no cash to buy groceries with (and because this was a highly focused storm event, there were no problems supplying the place with goods).

And this is far from the only situation were you're stuck without cash: for example banking systems going down means you can't pay with debit cards linked to accounts in that bank (a problem I've seen happen several times both here and when living abroad) and the banking payment system going down means you can't pay at all. The mobile network going down is also a problem because most electronic payment point of sale systems use it rather than landline. Beyond that there are all kind of issues linked to relying on a 3rd part entity for payments like the guy at the supermarket the other day whose just received replacement card wasn't activated so he he got to the till to pay a trolley full of shopping and couldn't.

In Engineering terms, cashless payments have a lot of external dependencies that cash payments do not, plus there is a natural "buffering" with cash (which you yourself can make deeper by having some cash at home) which doesn't exist with digital payments, making cash way more robust than digital payments when doing physically-present payments.

Hurricane Katrina, 2 weeks no power and no internet or cell service. The local store was literally giving the cold foods away, as the coolers didn't work, but they ended up getting a backup generator in for basic power to the lights and pumps, and they had like a mile of cars lined up to get gas, and buy dry goods and canned goods.

This was back in 2005 ya know, in a small town flooded in and struggling. Even the people running the store were struggling, they had to resort to taking a tractor to work. But we all helped each other, and the store was glad to sell whatever viable goods they had, for cash, and kept up with everything on pen and paper.