Obviously, for many of us it seems wasteful to just throw away unfed sourdough starter. But there's more it than simple frugality. The acidic unfed starter adds an interesting flavor to the sweet oatmeal cookie dough. The lemony glaze echoes that slight tang. The acidity also tenderizes the batter for a soft crumb. Wild yeast in the unfed starter gives the slightest lift to the cookie, making a lighter cookie that melts in your mouth.