in a small tribe everyone knows the value of each other's contributions to the community and can easily reward it proportionaly.  as the tribe grows, this becomes harder to track. verbal or written communication can be used to share value second hand, but that will soon not be enough. money is a language, a means of communicating the value you have given to another person or group. we can't fully understand the value of every service or good someone provided somewhere, but we can agree on a unit of exchange to represent it. to hand a cashier $5 is just a short way to say, "a portion of labor or service equal to the value of this item has been done, so i'll take it in exchange"
United States