Why is the first stage- chewing well - the “most important”?

It is the only part of the whole Digestive System over which YOU have complete control. In addition, the rest of the system depends almost entirely on how you handle this first part. The more the food has been processed by chewing being changed from being a variation of foods to a nutrient that the body can use as energy, the easier it will be for all the other parts of the digestive system to do their respective duties and functions according to the needs of your body.