No one has mentioned the 'pulling' of supplies by HQ units. This was a common use of HQ in WiTP and, I think, is also done in AE. Namely, HQ tend to attract supply movement towards them-makes resupply of units in front line combat more efficient
This can be critical by the way, including the part where command HQs try to pull 20k+ to their location.
Especailly, when you're trying to "pull" supplies to an inland base, where you're trying to operate bombers. Dumping lots of supplies onto an island or port (with a large AF for your bombers) is one thing. But if you've got an inland AF getting enough supplies to move inland to operate your bombers can be tough (altho this may be corrected in AE with the supply routing enhancements, I have no idea).
But real examples in my PBEM vs. Bilbow. Try to get (a lot of) supplies into Mytkima and Lashio and Ledo (all inland). Even if you hold the mighty port of Rangoon w/ 400k supplies, it's still tough to get supplies to these bases (and Ledo in on the India side). Now put 250x Commandos at each base, and watch your supply requirements go thru the roof. Except the supplies don't move. How to resolve?
AirHQs help -SOME-. They do pull more supplie than if none were present, which means you then fly more supplies into Chungking. But about half the time, the supplies are still red. How to rectify the situation?
Move a command HQ there (in my case, I had SEAC in Rangoon, and IndiCom Calcutta; I move them to Lashio and Ledo respectively). One other thing is needed however, I had to change the owndershp of the base (Lashio), to SEAC before it would pull 20k+ supplies.
So anyway, long story short (too late).
If you need supplies in a location, get a command HQ there,
and make sure the base is assigned to that same command HQ. Now you have lots of supplies. Maybe you're trying to fly supplies over the hump. Maybe you're trying to fly a bunch of bombers. Maybe you're pushing an offensive. Get your command HQ on scene at the nearest city, either for your bombers, or for your advancing armies to draw from that nearest city.
Again, doesn't really matter if it's an island or port hex, because you dump supplies to heart's content. But if it's an inland base = critical.