Second time through allows for nutrients they need that didn't get absorbed the first time. All tortoises eat feces. Many African savannah types eat hyena poop as it is high in calcium, they eat hookstock poop as the hoffstock does not process all the nutrients out, and can reach leaves the tortoie can not reach. Lots of good food quality in that poop. Many species of animal do this.
I fourth the poop eating. If I don’t see it before my red foot does he’ll eat it every time. He’s not a cage pooper but does in his soaks and he’s quick to scarf it down.