(imperfect) stamp of quality and amount
Gold was used directly, too, as ingots, and as bullion (check out the gold rush of 1849).
But coins show the insignia of the state (and usually the monarch) that minted it; this is an important symbol, and also a guarantee of purity and weight, at least nominally.
If the value of the coin come from the gold in it, why mint them instead of using the gold directly?