It allows you to get from house to car and vice versa without having to deal with the elements. It means that in winter you don't have to dig your car out. And it allows you to have windows that let light into the house on the same wall as the vehicle.
But if you're going to go that far with it, why not have an enclosure built?
And depending on the wind, snow and rain can easily get in there (same with hail).
It's like the builder started on a garage and just said freak it and took bongs hits instead.