This answer is not correct. It is actually quite rare to replace an atom or group of atoms on the same side of the molecule. It is much easier to either form some intermediate and obtain a racemic mixture or to have an inversion take place by attack from the rear.

Think about the energy required to replace someone that is sitting on a stool. If you have to approach from the front and pull them off and then move around them to sit down, that will take considerably more energy than is required to come from behind them and give them a little butt-bump and sit down...of course this assumes they will not be in a highly energetic state after they become the leaving group.