To expand on the answer by Albert:
Once upon a time, there was a mostly cash-based economy for the equivalent of $100 or less, and some sort of check for larger amounts. Then the "plastic money" became more and more prevalent -- first in the supermarket for the groceries, then at the bakery, finally at the hot dog stall for a quick snack.
But at first cash would simply become uncommon without being phased out. Small kids still get their pocket money in cash. One might give a coin to a beggar in the street. Things like that.
At some point, the government advises the citizens that a cashless economy is fine, but they should keep a little bit of cash in a safe place. Just as many governments advise their citizen to keep food and water for a few days at home, and candles and matches, and a battery-powered radio.
So sober, respectable citizens have an envelope with some mixed-denomination bills somewhere in their desks, and most still have a handful of coins somewhere.
When the blackout comes, those reserves are not nearly enough, any interesting stories arise.