All the concepts of COH are really funny including these fuel and ammo resources , these have been done not to suit reality but the balance of the game
Hence there is no use contemplating the purpose of each and every thing.....
Lts , and Captains do not drink fuel and neither are medic kits available in exchange for a Thompson round
Its just the simple design of the game that dictates this
COH is an RTS game where you will have to focus on how you fight rather than how best you can gather resources which is one of the main objective of many games like AOE
Hence the design is 70% balance - 30% reality
"There goes a joke about COH"
"Its only the reality part of COH , that prevents shermans from flying in to air" Ammo upkeep etc is a question of balance not reality
!!