Reduce vehicle and aircraft manufacturing costs
Due to the Middle-Eastern location of many of the battles waged with the GLA, Oil Refineries were a common sight on the battlefield. A commander from any faction could take advantage of these facilities by ordering one of his infantry units to capture them. To do this, he first had to upgrade the training of his infantry with the Capture Building ability, available from the Barracks (with the exception of the Chinese special agent Black Lotus/Super Lotus, who required no such upgrade).
Once captured, an Oil Refinery reduced the cost of vehicle fuel and, by extension, the overall cost of vehicle production by 10%. During a lengthy battle the money saved by the ownership of an Oil Refinery could be tactically important, and may alter the outcome of some important skirmishes.
Generals had to protect their Oil Refineries well, however, as they could be captured by the enemy or, should the enemy be more callous, simply destroy it.
The 10% cost reduction also applies to how much money is received upon destroying vehicles by GLA generals who have purchased the Cash Bounty Generals power (e.g., a GLA general with rank 2 Cash bounty only receives $108 for shooting down a Chinese Mig instead of $120 if the Chinese player captured a refinery).
On maps with multiple refineries, capturing more than one refinery has no effect.