Money or currency is a way to represent value
It is a medium of exchange
Could your boss pay you in cows? Could you take those cows and trade them for a car? Yes but this would be difficult.
So if your boss gave you a cow voucher “money” and you could trade that for a car, then you wouldn’t have to lug the cows around.
It facilitates trade
Why a US dollar, or another currency, has value is a whole other question







Or just roll them around in a bowl
Shaking is not required