A crisis prevents people from earning money. No money means nobody buys anything. Nobody buys anything means no company can now sell stuff, so no revenue. Companies start closing down, so there are even more people who cannot earn money.

The government can print money and inject into the system. Some people have money so they continue to buy stuff but maybe at a slower pace or less amount. Things also can get expensive but it is not a total collapse.

one of the big problems of the great depression was banks went bankrupt left right and centre, and took everyone's life savings with them. Also as a lot of banks generally hold mortgages in their portfolio, so when a bank collapses it means that mortgages all get fucky too.

So the mass printing of money meant that banks didn't collapse.

It also meant that a fucktonne of cash went into the hands of the uber rich.