The primary benefit of a monetary system of exchange compared to a barter system is the increased

flexability

Explanation:

A barter system is only as good as the items being bartered (or traded). For example - if I need a new car and am willing to trade my tractor for one, but the person who has the care does NOT want a tractor - we cannot barter. I need to find someone with a car that wants to trade for a tractor.