It takes energy to get out of bed, put your pants on, make a fire, scoop up some water, pound wheat grains with a rock, drop them in hot water and make bread, cut a branch off of a tree and make it pointy, run after a lion and throw your pointy stick at it and get some meat, sweep the old lion bones out of your cave, chase after your kids and bring them back to the cave, slap mosquitoes on your arm, get up in the middle of the night and chase the racoonosaurus out of your cave that's trying to steal your lion meat, and a lot of other things like that.
People have ALWAYS needed to do things like that just to live, so they needed energy every day, and I think they must have found a lot of dependable ways to get it.
This is just my opinion. I could be wrong, but I'm pretty sure.
