What is the tactical point of being in Afghanistan?

There is no real goal for our troops there, except to kill as many taliban as they can. But even then, why? The taliban are only killing our troops because our troops are right in front of them. We aren't exactly nullifying their ability to attack us. The locals don't mind the taliban and the Afghan government isn't doing anything about it because they just don't see them as much of a problem. Because of religious and cultural roots, the locals have no intent of moving out of their homeland nor do they care for modernization, which takes away that argument. The fighting there has no point... Does it? Will someone explain to me why the U.S. is there?