Why are sausages often stuffed into casings?

Sausages are typically stuffed into casings as part of the traditional food processing method used to contain and shape finely chopped and seasoned meat.1

Historically, this practice served as a functional way to package meat products. Casings can be derived from the internal organs of meat animals, or they may be made from modern synthetic materials such as plastic or reconstituted collagen. In the production of "skinless" sausages, a cellulose casing is used to hold the ingredients during processing; it is later removed after a water-immersion step forms a thin protein film around the meat. While many varieties are encased, some products, such as certain cooked meat specialties and luncheon meats, are prepared in loaf form without the use of casings.1