The adage “practice makes perfect” applies to the prepper as much as it does to anyone. In order to stave off panic in a survival scenario, it’s a good idea to be familiar with your equipment and supplies. It’s also a good idea to get a feel for how your body might react.
If you’re caught in an emergency situation, food may be scarce. And if water is scarce but food is plentiful, laying off the food until water is found will delay the effects of dehydration. So it’s a good idea to get a feel for what it’s like to go without food for a couple of days. Fasting is also a healthful way to eliminate toxins from the body.
Unless you have significant health issues (diabetes, etc.), a three- to five-day water-only fast won’t harm your health. But it will give you a good idea about how your body will react when confronted with a survival scenario in which food is scarce.