I think at the scale of economic systems, the system "moves" or develops toward an optimal expression of organization, and an optimal rate of increase of organization. This can also shed, however ugly it may be, a light on -isms like imperialism. Not saying that they are necessarily here to stay forever or that they are absolute, but they take hold, or amass influence, because something about them qualifies them as conducive to the forces that propel the organization of matter. These forces might express themselves by different rules/principles as they scale up in size and complexity.
Until we can find a better medium for this kind of growth than money, it will continue to reign as it does. By better I mean more conducive to self-organizing systems.
What function, precisely, does money serve for these systems? I'm not sure, maybe its just an optimized (more mobile, more quantifiable, more systematizable, etc) representation of value? Why, because it is largely dividual/divisible in more flexible ways than other material commodities?