Q: What is the geostrategic and geopolitical importance of Afghanistan?
A: Afghanistan is not so significant in itself, if we only consider economic interests. Of more importance are some countries situated near Afghanistan, especially those in Central Asia and Azerbaijan.
Afghanistan is also a traditional buffer zone, since its landscape is hard to penetrate for tanks and modern armies. It has prevented the expansion of the Eurasian Heartland Empire towards Eurasia's southern rim lands for centuries. It has protected the areas included in Pakistan and India today, but on the other hand, turning Afghanistan into a politically or militarily active area was used to destabilize Pakistan, or Central Asia, in order to alter the status quo, whatever it was.
Regarding oil, Afghanistan again forms a bridge or a barrier. As long as Iran is regarded as a hostile country by the US, Afghanistan forms an oil transport route from Central Asia to Pakistan. As long as there is war in Afghanistan, it remains a barrier preventing the countries of the Caspian Sea from benefiting from their oil. Wars in the Caucasus have exactly the same outcome. While this is the case, only Russia and perhaps China will have access to and hegemony over the energy resources in the vast Eurasian heart-land.
I think this is the main geopolitical importance of both Afghanistan and the Caucasus. It is the question of Russia monopolizing the geopolitical heartland, first and foremost. Considering the colossal weight of geopolitics and geopolitical thought in present Russian security thinking, these implications cannot be overestimated.