State of the Union Address Response

In his address President Obama mentions that the last the troops that were deployed in Iraq are returning home. Then he later says that some of the troops that are in Afghanistan are coming home also, but he doesn't mention why they are there. He is keeping troops in Afghanistan, but why? Why do we need to be there? I understand that we are there to show other countries that we are a force to reckon with but right now our military is playing refereres during their play time. It is costing us lives and money to stay there with minimal benefit to our country. I wish he would have mentioned why our troops are still there, why we are still pouring tax dollars and putting our nation into further depth. I just wish he would have expanded further.