Turning speeds the process along, and eliminates the outer "crust" of the heap, if it's not surrounding by something solid, but it is unnecessary if you do not have the time or inclination. This method is recommended for hot composting--turning refuels the bacterial fire. Hot composting is bacteria-powered; cold composting has a more diverse balance of bacteria and fungus, and some actually advocate for that process over the hot version, in part because it is better at conserving nutrients. But hot does, indeed, produce compost faster and probably more uniform. Cold composting is much slower, but in addition to the fungal breakdown process, it is a better host for decomposers, who could not survive otherwise.