It is either a " history of thought" type of motivation or maybe Brad just doesn't understand the reasons why economics as a whole (that includes New Keynesians such as himself as well as Chicago guys like Fama) turned away from historical and institutional approaches after WWII. I can really never tell with him -- sometimes he has some thoughtful things to say about methodology, and other times (say, when he is talking about why we shouldn't talk about Marxian economics) he just comes off as stunningly ignorant. See this piece:
I think this is a very insincere discussion of the theory. Or take his views on Polanyi, which I think are superficial (he gives a "supermarket analogy" for understanding the concept of embeddedness, which doesn't seem appropriate when applied to labor markets, thereby missing an important part of the 'embededness' idea):
In short, in reading DeLong's notes on why we should study economic history, he seems to appreciate the discipline as a tool for understanding "what really happened". That's fine if you want to point out in an ad hoc way the blind ignorance of most of modern economics to reality, but I contend that economic history is much more meaningful as an actual discipline. We should study economic history because it forces us to understand institutional processes, like how market integration occurred, or how the transition from Feudalism to Capitalism occurred.
Both of these examples are studied in the economic history classes at UMass because these are issues which demand more tools and a broader view of study than economics alone permits. Yes, on the way we at UMass still encounter a greater range of "facts" with which we illustrate theories, but more fundamentally we get to understand the social relations and (political, cultural) institutions that underly economic activity.
What we find is when we add social relations and institutions to the mix, economic activity becomes much more varied and thus much more beautiful as an object of study.