That is, from the viewpoint of U.S. imperial interests. The invasion toppled a longtime enemy, and served as a warning to smaller countries in the region thinking about greater independence. Iraq now exists as a balkanized, deeply divided country, that is susceptible to classic divide and rule strategies.
The invasion opened up Iraq to the Western private sector - especially in the oil industry. American and British oil companies were the first recipients of contracts to service and develop Iraqi oil fields. Of course, non-Western companies will also play a role, eventually. But the U.S. will be the dominant player by virtue of its military presence. It's worth remembering that Obama never promised to withdraw all U.S. troops from Iraq, just the main body of combat forces. Maintaining a large standing army in the Middle East is poor tactics, too expensive, and irritates the local population. However, there will be a U.S. military presence in Iraq for the indefinite future, working with the Iraqi military in a training capacity, and in small elite units fighting 'Al-Qaeda' remnants. This presence will give the U.S. crucial leverage, and contacts, as the military power structure is developed within Iraq. It's also the long-term tactic being attempted in Afghanistan.