Dutch Uncle
* Tertia Optio * Defend the Constitution
James Madison said it is to control factions (Federalist #10).
Good point but I see that as a subset of resolving conflicts between the rights of citizens. If a mill begins dumping waste into a river that downstream farmers and ranchers use, the latter will object; their right to not be poisoned versus the mill owner's right to dump waste wherever the fuck he likes. It's either war or court. Government provides the court.
There's also the point, as someone else mentioned about doing things locals can't such as provide for the group defense or organize building roads between states/major points of trade.