Why Geese Fly In A V Formation

Because it puts each bird in the most favorable position with regards to the slipstream of the bird in front of it.

That's also why geese will 'rotate the leader'. So everybody gets to rest a bit.

Other birds (such as the Albatros) that fly long distance on their own use the ground effect for much the same reason, to conserve as much energy on long distance flights as possible.