As an example of the difference, look at oil or bananas. Demand for those is significant, but because storage is inconvenient, holdings are low compared with total commodity flow.
Transactions are necessary but insufficient for something to become a store of value. For example, steel is sold in great amount, but it is not a store of value.
For example, let's suppose that I made a deal with Porche to buy the first car off their assembly line each year. I take it and store it away as an investment. Is there an existing reserve of 2018 Porches? No, I have the first one. Does anybody else do this? Doesn't matter. It's still a good store of value as long as people keep wanting 2018 Porches.
People could use steel as a store of value; it's just an inconvenient commodity to deal with because the value per gram is so low. You need to store a lot of it. But rarer metals will do fine.
https://www.statista.com/statistics/274684/global-demand-for...
Gold has been valued from time immemorial for its unique aesthetic and practical properties entirely aside from its value as a currency.
That's clearly distinct from, say, paper dollars, which indeed have value only because we agree that they do (and because they're backed by the robust efforts of a government, of course).