To make any currency system work, and this is much simplified, all that is required is that a majority of its users all agree that it is worth such-and-such.
I think that's quite right. Value is a relative term. If you imagine the very first coins made; a lump of metal such as silver of a fixed weight was stamped with an official mark. That lump of silver (a coin) was then the preferred
medium of exchange compared with an unmarked piece of silver of the same weight. In other words, a silver coin had a greater value than the same weight of silver bullion. If that wasn't the case, the use of coins would never have taken off.
Skipping forward to the Roman period, provided the coinage remained overvalued (or conversely bullion silver remained under valued) the system remained stable. The system could even take some de-basement and reductions in the weight.
Of course, the whole system went pear-shaped in the 250s. One of the main problems must have been that the 'silver' coinage could no longer pegged to the gold coinage
. This must have split the system into two economies, gold which could be used for long distance trade and the rest which could only be used much more locally.
It seems possible that the Sestertius, because of it's imposing and unchanged size was regarded as a an alternative to the nasty small Antoninianii - a case of 'user confidence' - because double Sestertii (if that's what they were) were produced at greater weights. Ultimately, of course, that didn't work out either.