Fairly sure the answer is: It allowed people to buy and sell goods in a wider market.
Think about it: without coins, people had to barter with mostly short-term goods, but coins held value over time and most people would accept it as payment. However, if you had to use, say, carrots, the guy you're trying to buy lettuce from may not need/want carrots, but he can use those coins to buy what he DOES need.
Hope I helped!
