Why would anyone fly a 777 to Afghanistan or Turkmenistan just to end up with a grounded 777 with no fuel worth mentioning?
I think that it's for a future terrorist attack.
This was thoroughly and elaborately planned, so it probably had enough fuel to make it to the intended landing spot, and then was taken somewhere else after a refueling.
What type of terrorist attack? Crashing it into something, of course, but perhaps loaded up as a dirty bomb first. A 777 can hold lots of chemicals or radioactive material. Iran, for example, has plenty of radioactive material thanks to no one stopping them from having it. The target could be Israel (easier to get to), the US (hard to get to, but perhaps not impossible), someplace in China (but probably only if it is loaded as a dirty bomb, otherwise, why fly it out of China first, but maybe the initial pilots were willing to hijack but not crash it), maybe Russia -- any place that is subject to Muslim attacks. My guess is highest probability is Israel or the US. The attempt doesn't have to happen any time soon. They could store the airliner out of the way for a while.