Here's Why:  

Since there is such a focus on eating a low-fat diet, people often think that any food that is low in fat is inherently healthy. This is not the case. For example, soda and hard candy have no fat, but they also have no vitamins, minerals, fiber, or other health-promoting ingredients. What they do have is sugar, and lots of it. A lot of sugar can add up to a lot of calories.

Eating foods high in sugar and calories can lead to weight gain. Being overweight increases your risk of chronic disease, such as heart disease, high blood pressure, and type 2 diabetes. Excess weight can also increase complications of other conditions you may have.

In addition, sugary foods often take the place of healthy foods, for example choosing a soda over a glass of skim milk or snacking on gummi bears instead of on an apple.