When you think about it why do we have to eat?
Same reason as why other animals eat, or have, sex, or breathe....because its natural to do so.
How much of the food/nutrition that we consume is just stuff that goes through us? In other words faeces.
Different amounts depending on how well you digest it and what you eat. I know it doesn't all go through me because I can feel certain beneficial effects very soon after eating healthy foods. Such as energy, better mood, sexual drive etc....can be also negative effects too, or neutral ones like sleepiness, sometimes its not welcome but sometimes its great.
I think with breatharians they just conditioned their metabolism to go very slowly, hence why they need minimal amount of food, but they still need it. Anorexics are also breatharians but are not healthy obviously, and don't have a good quality of life, because they didn't condition themselves properly, the few foods they eat are probably not the healthy ones...otherwise, anorexics would be healthy and long lived too, a perfect example that something can be done the wrong way and the right way.
Animals who hibernate don't eat while doing so. I think breatharians have achieved hibernation body state without having to "rest" all the time like hibernating animals.
I personally have a very quick metabolism and would be a horrible candidate for breatharianism.
Maybe eating mey make me die a little faster, lol, but really thats not a concern, I'd rather receive the pleasure of food then live a little longer. Plus I think with breatharians their metabolism becomes so slow that if they happen to eat a lot of food they will gain weight like crazy. I don't like that idea because IN CASE you want to eat lots of food now you can't...they may never want to of course, but its putting yourself in a ultimate condition, which freaks me out a little because I prefer to be able to consume something in the case I want to or have to and not suffer a great deal. However I do have respect for these people to be able to achieve that and have such a great body composition, its quite amazing!