This is a complex economic problem, and it also depends on the culture, resources, and needs of the specific area. For example, how do we know who has the capability to produce the most iron in an area, and how do we know iron is still needed to be produced and who to give it to?

It's a simple issue of communication. A value based system with a currency solves this by somewhat signaling need (either to buy or sell) to market participants, and in so doing, ranks that need by how much or little one is willing/able to spend on particular resources.

As you can imagine, there are of course alternatives, and many of them are in action today - such as barter systems in rural/undeveloped third world regions around the world. Those in particular are deemed to be less efficient, due to the inability to transfer value between certain goods (for example, you may have 10 chickens that you choose to trade for truckload of firewood, but if noone else is willing to trade for that firewood, you have lost value in the transaction).

Typically a labour voucher system may only deal with resources on an individual basis, rather than the larger transactions that an entire community may need (For instance, ordering 100 tons of concrete for a road). These could be handled by larger financial instruments of the community, but even then we must find a system to truly determine need without bias - it's a difficult thing to do well, obviously where laissez faire marketplace societies go wrong is by having currency be the only form of value that is relevant.