In the Enlightenment there was no understanding of why emergence rather than dissipation should occur in any particular situation, and so they simply inferred that potentials were suitably related from the fact of their development. In contrast, relations that resulted only in stagnation or disspation were called non-rational. That is why, for example, our nation's founding fathers were so reluctant to give propertyless wage earners the right to vote and, by extension, to give Black Revolutionary War veterans pensions in the form of land. Historical study, then, is the search for one's roots. History therefore becomes relevant to the extent one is studying one's own particular heritage, what is more recent, less culturally mixed, and more racially pure.
Relevance ends being a function of empirical proximity.
This essentially parochial view of history can, however, be extended to world history. However, this simply applies the same approach to others, for they are a product of their past just as you are. This essentially mercantile understanding of history is found in the typical world civilization textbook or in World Systems Theory, where an understanding of the inherited traits of the world's peoples lends itself to their rational interaction. The great historian Geoffrey Barraclough articulated this point very effectively. We must study their roots just as we study our own so that we understand how to connect and build a better future.