Some bakers add the sugar to give the yeast cells additional food.

          1 package active dry yeast
          2 cups warm water
          2 cups flour
          2 tablespoons granulated sugar

          Using a stone jar or crock, dissolve the yeast in the warm water. Add the
          flour and sugar. Place mixture in warm place for 3 to 4 days or until it
          is bubbly and smells sour. Refrigerate it.