If you don't pay attention while you pour, salad dressing can easily add up to a lot of calories. After all, most dressings are made with oil! It makes sense, then, that a low-fat version could help you cut back on your calorie intake.
Still, watch out for low-fat dressings. Some make up for their low fat content by adding extra sweeteners and starches. Hidden Valley Original Fat-Free Ranch, for example, lists corn syrup as its third ingredient, and then goes on to feature a laundry list of artificial colors and preservatives.