If there is a medium of exchange, it opens the door to more and easier possibilities. Say I have toy car for trade and in exchange I need a toy bear. And lets also say there is a toy bear for sale for $1. So in a trade only scenario, I have to find some one who has toy bear who also wants to trade it for a toy car (difficult) or make a series of trades to end up with the bear I seek (even more difficult). Or I can find someone who wants a toy car and will buy it for $1 from me and then I can just buy the toy bear with the money or medium of exchange (easier).