Why is this the answer?

The answer is war because this is a very thoughtful riddle that uses tricky word pictures to describe something serious that happens when people or countries fight each other. War is not a good thing, and the riddle shows us how strange and sad it is. Let's break down the first clue: "A harvest sown and reaped on the same day in an unplowed field." Usually, a harvest is a good thing that farmers work for, but in this riddle, the "harvest" is all the sad results of fighting, like damage and sorrow. This sad harvest is "sown and reaped on the same day" because wars can start very suddenly and cause bad results right away, unlike a farmer's crop that takes a long time to grow. It happens in an "unplowed field" because war is not something natural or prepared for; it just starts in places where life was normal before. The riddle is saying the results of fighting appear very quickly and without any good reason or preparation. The next part is: "Which increases without growing, remains whole though it is eaten within and without." This part talks about how fighting spreads. When a small disagreement turns into a big fight, the amount of damage and fighting ("it increases") gets bigger, but the idea of war doesn't actually grow like a person or a tree ("without growing"). It "remains whole" because even when the fighting is happening everywhere and hurting everything ("eaten within and without"), the terrible, complete problem called "war" stays exactly what it is. It means war is always a whole, awful problem that doesn't change its nature, even if it’s breaking apart everything around it. Finally, we have: "Is useless and yet the staple of nations." War is truly "useless" because it hurts people, destroys homes, and causes a lot of trouble instead of solving things peacefully. It creates a lot of loss and sadness. But for a very long time in history, when people had big disagreements, they sometimes chose to fight. This fighting became a very common, or a "staple" part of how different groups or countries dealt with each other. This riddle is clever because it makes us think about this big, serious thing and how it is described with such simple, puzzling words.