It was quite common for episodes to feature meal times and the cast eating meals as well. But the meals also needed to be somewhat authentic to the time period the show appeared in, which limited many of the foods common in the 70s to be used on set.
Beef stew was a common meal served, and on set they used Dinty Moore Beef Stew to serve and eat.
Chicken legs were another common meal, and for those times, they would use Kentucky Fried Chicken (KFC) to serve to the cast, which is often easily identifiable as being from the fast food restaurant when it appears on screen.