A* runs faster if you have better path cost estimates. To get the shortest paths, you want an estimate that's as close to the true cost, but not greater than it. You can precalculate to generate good estimates. Traffic makes the costs higher than normal, so those precalculated estimates are still useable (if you raise the costs because of traffic, the estimates continue to be lower than the true cost), and still much better than what you get without precalculation.
