Sure, it helps keep Russia out--well, less so recently, it seems.
And it is a power projection platform for operations in an arc of crisis from west Africa to Central Asia (and the Arctic now)--as even President Obama is discovering with the rise of ISIL/ISIS.
And it is a means to make sure that whatever real military capabilities NATO states have can operate with American forces.
But at the most basic, NATO helps keep Europe from becoming a hostile shore.
It is in our interest to keep Europe--a major center of industry, people, and technology--from falling into enemy hands. If a hostile power controls western Europe, we are potentially screwed.
We fought two wars to stop Germany from being that enemy power and one Cold War to stop the USSR from being that power.
Now we have to worry about Russian ambitions again.
There is also the worry that Europe could be
With so many Europeans joining the jihad in Syria, Europe could (again) become a sanctuary for terrorists to attack us--but even bigger--if European governments reduce their cooperation with us in the mistaken belief that if they cover up and don't make eye contact with the nutballs, that the nutballs will move on to hurting only America (and Israel).
And, as I've long written, we need to worry about the European Union being that potentially hostile power if it ever builds upon ever-expanding cheese regulations to become the real government of Europe, leaving national governments as mere executing bodies for EU power delegated to them.
NATO is our institution for keeping us in Europe and slowing the rise of the EU until Europeans come to their senses before it is too late; and to ward of Russian efforts to rebuild the empire.
NATO isn't charity on our part. This is a core American interest.