My sister believes that the cleanse gives certain parts of your digestive system a break. Liver, gallbladder, pancreas, stomach and intestines. Since you are only taking in simple carbohydrates they are easily absorbed in your mouth and stomach. So your body gets a rest from digesting proteins and fats and complex carbs. The liver and gallbladder especially play a major part in digesting fats and proteins.
Hope that helps.
p.s. My sister thinks swishing is not as bad as chewing / swallowing.