We learn that it would be the price for the War Rig's safe passage through the biker territory, as negotiated previously by Furiosa. However, the deal couldn't obviously be a part of Immortan Joe's original plan.
When Furiosa and the Rig are introduced, we learn that the original mission was to take water (aqua cola) and mothers' milk to Gas Town and Bullet Farm. Nothing is said about the fuel pod however. Its being full seems out of place because why would anyone bring fuel to Gas Town? They have plenty of it!
The deal was most likely to bring the fuel back from Gas Town in exchange for the food products, so in that case the fuel pod should be empty. I doubt that the same tanker would be used to haul water and fuel, so the pod being empty to bring home guzzoline would be logical.
I also doubt that it would be used for refueling since all the settlements are relatively close to each other. The Gas Town is clearly visible from the Citadel and the Bullet Farm cannot be much further, since both settlements' war parties arrive roughly at the same time.
Is it possible that Furiosa somehow managed to sneak 3000 gallons of precious guzzoline into the pod under Immortan Joe's watchful eye? It seems unlikely since it must have required some cooperation of a couple of Immortan's subjects and the movie does not hint at any conspiracy. It would also be fairly easy to notice the difference between a full and an empty pod (weight difference could be noticed by the lift operators, it would also influence the shape of the tires).
Or did the pod serve some other purpose, which I fail to recognize at the moment? Or is it just a
gaping plot hole?