The acid from the vinegar/lemon juice helps stabilize the egg, causing the whites to quickly coagulate around the yolk.
If you need to poach eggs for a crowd, this recipe will work for up to 8 eggs. To maintain the correct shape for the poached eggs, still put only 4 eggs at a time in the water to cook. Crowding the pan with all 8 eggs could cause breakage or misshapen eggs.