I think they’re used to cook food items quickly, especially roasts or other large portions of meat...not positive. Also used to sterilize canning jars and lids when canning food.
A pressure cooker turns the toughest roasting hen or chuck roast into falling off the bone tender, usually in about 20-25 minutes. Introduced after WWII, it was the microwave oven of its day.
Also works great for boiled peanuts, as in 45 minutes instead of four hours.