[RobertoG]

"[..] come to a counter at the mint and deliver their metal[..], and they would be paid back, within a few weeks, in newly minted coins of the same metal they brought in. They always received back less fine metal than they brought in."

If the quantity of metal was the origin of the value and they weighted the coin when using it, why they need the mint in the first place? why not to use the metal directly?

[reidacdc]

I'm really guessing, but perhaps the quality of actual metal is difficult to assess, but a minted coin with the right markings carries a promise of correctness. Counterfeits are still possible, of course, but less likely.