How are vegetables important? They provide little to no energy.
That's why they are important. :)
On a serious note, there is nutrient density.
http://en.wikipedia.org/wiki/Nutrient_density
Most commonly, nutrient density is defined as a ratio of nutrient content to the total energy content. Nutrient-dense food is opposite to energy-dense food (also called "empty calorie" food). [...] Fruits and vegetables are the nutrient-dense foods, while products containing added sugars, processed cereals, and alcohol are not.
This is why replacing added sugar with anything that contains at least some vitamins and/or minerals is beneficial.
Looking through history, humans only had fruit available a few months/weeks of the year. Nowadays we can get all the fruit we want.