While we know also they use this primitive currency for salary, so practically this is their income. We can think Japanese koku for example, it just a form of daily income but they not really use it for real transaction, because 1 koku stand for 278.3 litres. Of course they not carry all of these rice in their pocket to go buy their daily stuff, while samurai who get a salary around 30 koku per month can be consider as poor samurai while a Shogun can have an income like 1 million koku per month. But the use of something that less functional like arrow head, shell, also metal in bronze age, it might be stand for something like modern paper money stand for. Because it have value when the society give value it don't have a special intrinsic function or value, the value depend on the consensus of the society to put or not to put faith on that object, just like paper.