As makers of fruit preserves, jams and jellies know, if enough sugar is used, it helps preserve fruits. Yogurt manufacturers also take advantage of this storage-extending ability. Often, though, even more sugar than needed is added to satisfy the sweet tooth of the typical customer.
It is a sad fact that many lovers of fruit-flavored yogurt think they are eating a particularly healthy food. If you offer the same people bread smoothened with jam, they probably wouldn't give it too high a nutritive rating. Yet, fruit-flavored yogurt is essentially yogurt with jam or preserves.
What tricks people into thinking that they are consuming a healthful product is the phrase "100 percent natural" printed on the container. Sugar is an empty calorie food. It has virtually no nutritive value and can harm teeth and other parts of the body, yet the manufacturer can use the phrase because the sugar is "100 percent natural" by definition. So is a rotten apple.
If you enjoy yogurt, eat it plain or add your own fresh fruit. Unless you plan to store your yogurt mixture for a few days, there is no need to add sugar to preserve the fruit.