Hillary Clinton's inclusion of Azerbaijan in her current round of diplomatic visits, which also included stops in Poland and Georgia, reflects the need to balance the U.S.-Russia reset with symbolic reassurances to regional friends and allies. In particular, the Georgia and Azerbaijan stopovers underline the increased importance to the U.S. of good bilateral relations in the Caucasus and Central Asia. The reason? The Northern Distribution Network, the supply lifeline to U.S. and other NATO forces in Afghanistan, comprehensively covered in this CSIS report (.pdf).
Azerbaijan is part of NDN South, the back-up route that starts at the Black Sea port of Poti, Georgia (see good maps here). Non-lethal goods are transported to Baku, where they are loaded onto ferries to cross the Caspian Sea, before continuing on through Kazakhstan and the Uzbekistan-Afghanistan border by rail. Military supplies are flown across Azerbaijan's air corridors.
It's no accident that NDN South bypasses Russia, the main NDN corridor for resupplying NATO forces in the Afghan conflict. This reduces NATO reliance on Russian cooperation, and represents an alternative to the old rail link that, having once supplied Soviet forces in Afghanistan, was revived by Moscow last year to supply the Americans.