A traffic network's purpose is to move people from point A to point B. If you double the lanes and they fill up, you've doubled the throughput. If traffic doesn't move any faster, it's because the old traffic jams were a bigger problem than was visible. You've still alleviated the issue by allowing twice as many people to reach their destination in the same amount of time.
There's an argument that assumes that building more lanes does not increase the number of people who travel, only the number of people who choose to do so by car. Then with fewer lanes, the same number of people still get from A to B but on bicycles or using public transport instead of cars.
