Similarly to traditional financial markets, an asset is only worth something if someone is willing to buy it from you. If no one wants to buy your dollars at a specific exchange rate to another currency, then your dollars aren't worth that much. The reason why people would want to buy your asset, is due to some utility that it provides; its purchasing power, store of value, potential appreciation, etc. There are tons of physical assets in the world, such as currencies, precious metals, stamps, houses, cars, and so on. People choose to hold what they think has the most utility for them.