Kiyotaki said:
How come we use currency, what if everything was free and done by charity workers?
Currency is a measure of value. We all attribute an equal value to it, making it much more standardized than, say, a barter system.
"I'll give you five chickens for your cow."
"I don't want chickens. I want pigs."
"But all I have is chickens!"
"Oh fucking well."
Behold the world without currency above. Where chicken man will never be able to enjoy a steak or a glass of milk. Now, say we add currency in the form of gold coins to the world. The cow man won't exchange his cow for chickens, but he will sell the cow for 15 gold coins. The chicken man sells his five chickens for 15 gold coins and then buys the cow. Everyone is happy.
Simpler metaphor: 10,000 cans of soda and a car have about to same value. But you can't just buy a car with 10,000 cans of soda. You can, however, sell the soda, and buy the car with the money from the soda sales, because both the seller of the car, and the buyers of the soda, attribute value to the money.
Now, on why the world can't just live charitably. Because exchange allows for a higher standard of living due to the interdependency it creates between people. Say, there's a doctor. He treats people, cares for the sick, heals injuries, etc, all for free. But, how does he get water to drink? Or food to eat? He can't farm food or mine for water himself, because he only has the skills to be a doctor.
Instead, he exchanges his services as a doctor to attain the things he needs. Now, not everyone in the world needs a doctor, and not everyone who does need a doctor is able to give him the things he needs or wants. Here comes standardized currency to the rescue. The doctor sells his services for coins and then uses those coins to buy the things he needs. Currency acts as a middleman for the exchange of goods and services.