The social benefit to storing value is de-risking against the future (2). I keep some money in my bank account so that if I need to buy food for a month with no source of income I can(1). If the money in my bank account buys a third as much food tomorrow because it lacks stable value as a store, that's bad for society, because if I'm going to starve to death, I might as well use the remaining energy I have to burn some stuff down or eat the rich.
(1) Ideally, I of course live in a society that doesn't consider it acceptable to let people starve to death and has a secondary system for routing food from supply to necessity without demanding I exchange stored value for that food, lest I die because I have no measurable stored value. I'm using a simple model here, and we can probably move the particulars around to find something that a society doesn't provide which is considered valuable enough that people store up private wealth to acquire it (and lacking the ability to do so, would resort to burning the system down).
(2) There is probably an interesting question in whether such privatized de-risking against the future is actually a societal benefit (because of the risk that it becomes simple hoarding). Is that risk why you have discounted value-store as a societal benefit?
Money should have a predictable value, not a stable vale. That's why most economies target 2% inflation. If money had a stable value, then saving would be riskless, until the accumulation of saved money increased would put increased pressure on the expected GDP until that "stability" collapsed catastrophically (as in the Great Depression)
> I keep some money in my bank account so that if I need to buy food for a month with no source of income I can.
Well, that thing in your bank account is not money, but a complex derivative of it, with different risk and depreciation profiles.
Anyway, some money that you can easily receive from the other side of the world, immediate convert into local currency, and move away without seeing any of it again would be very useful, provide a lot of value, and wouldn't require any value stability if it was created at the moment your peer started the transaction and destroyed as you finished it. Ditto for something you could send very small amounts over the net.
The thing in my bank account is enumerated in US dollars and backed by FDIC insurance against theft, fraud, and collapse of the institution storing and tracking it causing the quantity of such enumerated dollars I have changing. I agree that it has different risk profiles than "A sack of greenbacks under my bed," but not different in a way that makes it practically not-money.
It satisfies the three basic functions for all practical purposes. In fact, it does so better than BTC (in the sense that there is no FDIC-equivalent to protect me if theft, fraud, or collapse of some holding institution tracking my BTC wallets for me divorces me from my BTC).
Yes, storing value is useful, but not storing value doesn't make a currency useless. You can always store value in other ways, by purchasing assets - such as food, or another currency.
It's worth noting that people tend to like to do this anyway, even when the currency is already pretty stable - because a house you can rap your knuckles against feels like a more secure store of value than numbers in a bank.
Money is designed (by central banks) to depreciate at 2%/yr. Most erstwhile "durable" goods depreciate at a faster rate. Your house certainly does. Heck, taxes and insurance alone are almost 2%, before you even look at physical depreciation, which is generally estimated to be at least 1%/yr.
(Speculative assets like the land it sits on might appreciate far more to compensate, however.)
I wouldn't say "useless" (it's clearly usable as an agreement between two parties that some amount of value was exchanged).
But it's less useful than a currency where the observed exchange of value between two parties implies that most other parties agree value was exchanged. If you give me 2 BTC today but tomorrow the value of BTC has fallen by 90%, it's almost as if we never exchanged value in the first place. It's certainly not as helpful to me as if you give me $18,500 dollars today and I can spend them tomorrow.
Failure of a currency to act as a store of value damages its utility as a means of exchange, because the amount of value exchanged is imprecise over time.
(1) Ideally, I of course live in a society that doesn't consider it acceptable to let people starve to death and has a secondary system for routing food from supply to necessity without demanding I exchange stored value for that food, lest I die because I have no measurable stored value. I'm using a simple model here, and we can probably move the particulars around to find something that a society doesn't provide which is considered valuable enough that people store up private wealth to acquire it (and lacking the ability to do so, would resort to burning the system down).
(2) There is probably an interesting question in whether such privatized de-risking against the future is actually a societal benefit (because of the risk that it becomes simple hoarding). Is that risk why you have discounted value-store as a societal benefit?