While the U.S. occupied Iraqi the Iraqi military was mostly refitted with U.S. weapons and equipment. That makes them a U.S. customer for arms, spare parts, technical services, etc. They can get a lot of this stuff from third parties but it cost more. Under Bush the plan was to turn Iraq into a trade partner so they could buy American goods with their oil money. That is happening anyway but by pulling the combat brigades out Obama has invited Al Qaeda back in. Al Qaeda can foment enough tension to get the factions fighting each other and everything goes to hell again.
They have to leave some time. Everyone knew this would happen. I would not want to see our troops over there forever, like Korea or Japan. I think the cost of that would be far greater than any gains made by trading with them.