"Digestion takes a lot of metabolic energy. After you eat, most of your metabolism is diverted to digestion. That's why you're sluggish or even sleepy after a big meal. (Digestion mostly stops if you're active, by the way, as that energy is diverted to moving around. Digestion would re-start once you start relaxing again)
Another process that takes a lot of metabolic energy is autophagy, which is the destroying of old, worn-down cells (which may have defects in their DNA and other operative machinery), eating them up and stripping them for spare parts like a burnt-out car, and making new cells (with correctly coded DNA) in their place.
When you digest, your body doesn't bother doing autophagy, as it's of lower priority. So going for long periods without any ingested food lets the body spend its metabolic energy on autophagy for a change."