I get the point of this in areas where it is Very Cold, like Canada. In some places parking your car, getting out, going to the atm, walking back to the car, and getting back in could a) get you very cold and b) mean you have to scrape the windscreen. So I can see that drive through banking is useful in those places.
But seriously, on the West Coast, it isn't that cold. And without that excuse, I can't think of another except laziness - anyone want to fill me in?
Apparently in Louisiana they have drive-thru liquor stores. That's something I gotta see.