Brad DeLong on Why Should Economists Study Economic History?

Most important, every piece of economic theory is ultimately a piece of crystalized history. And you have a much deeper and more sophisticated knowledge if you know the history that led people to think that elaborating these particular theories was worth doing. If you just do the crystalized stuff--well, there is a sense in which your thought processes are then on crack, unable to properly process and reflect on the systems of analysis you are using...

Second, just as every piece of theory is ultimately crystalized history, so every individual historical narrative or judgment is based on a web of implicit social science theories. And your knowledge of the past is inadequate if you do not understand your implicit social science theories critically enough to be expert users of them.