For example, several people have no money, but want to play a few games of mahjong, and then make an appointment for a card of poker to represent one dollar. Poker then generates a dollar value. The value it carries at the moment is the mutual promise of several people, and its function is to calculate the quantity.
In today's digital age, the immutable properties of the Bitcoin database accurately carry the mutual promises of Bitcoin holders, which can quickly calculate the quantity and transfer each other.
As a final note, Bitcoin is hard to come by, and if 21 million people buy one each, they will never be able to buy it again.