Vitamins are needed in only very small quantities because they are not very important in metabolism. The energy content of vitamins is so great that we need only small amounts of them. Excesses of all vitamins are excreted in urine. Many water-soluble vitamins are coenzymes. Food items are retained longer.
Specialized regions are possible.
Digestive enzymes can be more specific. Extensive branching is possible. Intracellular digestion is easier. A digestive juice with a pH of 2 probably came from the mouth. Most nutrients are absorbed across the epithelium of the colon stomach.