Currency has two worths. It is worth something as a printed piece of paper and it is worth something as a token. Both of those ultimately representations of human effort. If I tax you 1 groat per working hour and pay 10 groats an hour for your time then you can automatically price things in groats. If you print Somali shillings and that takes an hour to get 1000 shillings then you can price in Somali shillings. Because you are unlikely to exchange your hours worth of currency for less than an hours worth of effort from somebody else. That then ripples down the chain. Tax is a sufficient and reliable way of ensuring a demand for a token in a currency area. But currency can circulate for other reasons too as we see with Somalia.