Earlier quoted context omitted.
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 so…
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.
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.