I learned about this a few years back so here goes / I could be way off...

When originally constructed, concourse H did not offer FIS/international arrival capability. The upper level was nothing more than a walkway to the gates further out on the concourse (perhaps with moving walkways) but the rationale for having such a setup always seemed odd to me. With the construction of the south terminal and concourse J (and addition of FIS), a number of the gates on the one side of the concourse (H2/4/6/8/10) were modified to allow international arrivals via the (now sterile) upper level.

I never knew there was a lower level as well! No idea what that could have been used for