Actually there is a very clear purpose. The way things were going in the middle east, you had dictatorships that are keeping their people in ignorance and poverty, firing them up about the US and Isreal in order to distract them and keep their medieval way of life going.
The world is now too small of a place for that to be OK. It's to easy for the problems of the middle east to spill into the rest of the world.
The only areas of the world that have some semblence of peace between them have two things.
1. They are democracies
2. They are trading between each other.
By making Iraq a self-governing democracy, the hope is that this will spread through the middle east, and that these other states will become more democratic. There is recent evidence that this is exactly what is happening.
In fact, in the long view, 30 years from now, it may very well be that this is seen as a turning point in world history when the middle east begins to turn itself around and become peaceful stable democracies, as opposed to a festoring sore of trouble in the world.
