Bitcoin at least has a utility built into it, facilitating transfers and 100% guarantee that limited amounts of btc can be created.
Fiat currencies and btc are not the same but the underlying principle is the same. Demand and supply. If there is no demand its value will plummet.
2007 crash cause swiss frank to sky rocket in value. For no other reason then international crowd started buying franks to store their wealth in it not USD.