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.
Bitcoin is valuable because it has shown, thanks to the underlying blockchain, that a decentralized digital currency is possible. As long as there are enough independent "watchers" of the blockchain system then there is an inbuilt protection against attacks on the integrity of the blockchain, making it a global system based on transparency. That leads to another point - there is no artificial injection of unknown funds into the system, because every coin is traceable and this in another corruption reducing benefit. The last reason I'd give is the fact that lots of greedy banks and financial institutions around the world charge excessive fees just because they can, or were traditionally able to get away with it, so anything that can cut them out of the equation can create huge savings for the end users.