I'm not a 100% sure about this, but GPS might be used by some apps that use location services during background updates. For example, a weather app/widget. I keep mine off anyway, and turn it on whenever required, even though I have location services turned off for most of my apps (I just keep it fixed as my home city as I rarely travel).
As far as using a smart action for it is concerned, there's no trigger for applications. But if you navigate only when driving and use a car dock or a bluetooth device while driving, you could have it as a trigger to turn the GPS on. That way you won't have GPS on all the time, but if you ever need it while driving, you'll find it on.