It is very difficult for the body to digest carbohydrates such as grains and vegetables without some fat (oil, ghee, unsalted butter, etc). The fat acts as a fuel to break down and digest these foods. It prompts the gallbladder to squeeze out bile which is necessary for good digestion. Otherwise, these foods move through the small intestines way to fast (mostly unabsorbed) and end up in the large intestine where local bacteria begin to "digest" them. They ferment and produce toxic gases and sticky stools. Besides, these fats increase the taste of the food, important for good digestion.