War prevention. Allies are expensive, but wars are drastically more so. NATO has the capabilities to survive on it's own. The problem is that, if we leave, we instantly become an enemy to many countries who would no doubt attack locations we have interests in just to fuck with us.

No good would come from leaving.

It's like being an atheist living in a crappy religious town. You have to go to church so the neighbors don't egg your house. Sure, you could leave, but it cost so much money, it's cheaper to pay the church donation.