Physical Digestion is the mechanical breakdown of food from large lumps to smaller lumps where as chemical digestion requires enzymes to break down the chemical bonds in the chemicals that make up food, eg proteins are broken into amino acids. The processes differ as physical digestion is acting on lumps of food but chemical digestion is the action of enzymes on the bonds inside the food.
Physical digestion takes place in the mouth with chewing and in the stomach where food is mixed with acidic gastric juice until it becomes chyme. This is needed as it increases the surface area of food available for the enzymes needed for chemical digestion. Without physical digestion, food would be too big for chemical digestion to occur quickly.
Without chemical digestion, food would not be broken down into particles small enough to pass across the cell membranes of cells lining the gut. Chemical digestion occurs in the mouth (amylase breakdown of starch), Stomach (pepsin to break protein), and small intestine (sucrase, amylase and protease to break carbohydrates and proteins. Lipase to break fat). In chemical digestion, the different areas contain different enzymes that require different conditions. eg pepsin requires acidic conditions of the stomach and won't work in the alkaline conditions of the small intestine.