Travelers, for various reasons, prefer to carry their own snacks on flights. Shorter flights may not offer any beverages or snacks. Or, jet setters may have special dietary needs, be on diets, or have health concerns related to food and drinks.
Rules and regulations make packing food for flying more difficult these days, but travelers can still make choices when it comes to what to eat when on a flight. It is, however, a good plan to check various airline sites online beforehand and double check, since rules change over time and some edible items are iffy.