It identifies you as an “official” traveler on US government business. Kind of gave you a veneer of “respectability” (if that is the right word), and usually the guys at immigration left you alone and didn't ask purpose of your trip (plainly it was for US government business), didn't ask what you were doing and why.
Oh, and it did get you into places where the usual “tourist” passport could not because of some sort of travel restriction or limitation.
Nice!