You might be on a flight where no food is served.
You could even be stuck in the secure gating area of an airport
having to wait more than an hour for your flight which has been delayed and where even a cup of Yoplait Yogurt will set you back $1.78 plus tax. (something you are accustomed to pay $0.65 at your supermarket)

You could get hungry at an unexpected place and time.

It is not at all inconvenient to carry with you on your carry on baggage or tote bag a few of those so-called health bars or high protein "candy bars" available at Whole Foods, GNC, or even at your local supermarket.
These can often be lifesavers should you find yourself hungry in unfamiliar surroundings with no access to food. They are rich due to their enriched protein content and are usually satisfying substitutes for breakfast or lunch. It would also be good to carry a couple of small apples for dessert and if traveling in places like Mexico, carry some bottled water.