Why did whales lose their legs?

I wonder about evolution sometimes. I know whales evolved from terrestrial/amphibious four-legged mammals some number of eons ago. Eventually their front limbs adapted to become flippers to meet their increasingly aquatic lifestyle, and the hind limbs atrophied, shrunk, what have you. But why? I get the flippers, but what advantage would smaller hind legs serve over just having them dangle out there? Similarly, why did our faces change so drastically from those of apes? Do smaller ears and snouts really matter? These things don't seem to make a difference, at least in theory; and isn't natural selection all about the differences between the well-adapted and the non-?. Please, some biologist explain this to me...