> I don't understand what legitimate value ordinary currency produces. Can anyone help me understand?I'm not sure what "legitimate value" means in this context, but the most terse way I can think of to answer what I think is the spirit of your question is that currencies can be used to pay taxes and so there is an inherent demand, if geographically limited, for them; since you have to pay taxes, it makes sense to settle debts denominated in that currency because you have a use case for it going forward. This inherent demand creates stability--and yes, everyone loves to talk about hyperinflation but at the same time hyperinflating currencies are usually those of governments who are not long for this world!
If you want to go further, I would submit that a modern currency is one where the supply of it can be controlled in response to other economic factors, and my personal position is that that control turns crashes and depressions into dips and recessions. But this is an opinion, and reasonable people could disagree. (Though candidly--in my experience, few who do, are.)
Cryptocurrencies instead function more as commodities, and as they have few enough actual uses that are not by and large self-contained ones that require a certain amount of willing participation for them to have value at all (whereas gold is pretty and doesn't corrode, you can eat wheat, etc.) they are functionally inherently speculative vehicles. They lack any sort of external stabilizing factor because nobody needs them and so as a "legitimate currency" I can't see why anyone would ever tack on the phrase "legitimate currency" to that sort of virtual porkbelly.