Today's Economic models are purely mathematical constructs, derived from theories about the economic system. Unlike their analogs in physics, electronics, chemistry, engineering, and computer science, they are not derived from detailed study of the structure and behaviour of the components of the economy. We know this, if only because there currently exist no accurate descriptions of the implementation of the modern banking system, outside of this project.
Even the fundamental supply and demand model is incomplete. Other reasons for gold's price - or any other item - to change, are a variation in the supply of money, or the supply of credit.
Threadneedle simulations allow us to explore these kinds of issues very easily. Even simple market based simulations show immediate impacts on the price level - and effectively failure by the market system - to determine prices if there is insufficient money, or too much money concentrated in to few agents.
The value of agent based simulation for economics lies in finally being able to construct useful working models of the economy which can be used to test and evaluate theories before they are used on society.