It buys us fear and respect of our awesome capabilities
The US has all but openly declared the objective and intent of overthrowing the regime of Saddam Hussein. And now we'll show that we can, and we will, execute the mission, despite any and all opposition. One could argue the wisdom of such unilateral action, but it'll definitely send a very powerful message - America is unchallenged as the world's sole superpower, and can and will act in the national interest despite any foreign opposition to it.
If a Saddam Hussein in possession of WMD truly poses a threat to America's security, then we must take him out. If anyone else in the world has a problem with that, tough sh##.