If you need to cook something long and slow, you would simmer it rather than boil it. An example would be a stew that typically uses a tougher cut of meat. Simmering the stew would tenderise the meat by a long slow cooking method. Boiling it would cook it but the meat would be tough. Some foods such as sauces may curdle if cooked too quickly and some vegetables may disintegrate if boiled.