By stirring the wort really well before pitching the yeast, you are giving the yeast a good oxygen supply with which to kick off the fermentation. Once the yeast is added it will drop into the wort solution and begin preparing to ferment.If you stir after adding the yeast some of the yeast cells could be left clinging to the side walls above the liquid line and non-viable for the fermentation. Each cell is valuable, so if you keep them all in solution you'll have a better resulting fermentation.