Mine have feed in the coop, they free range inside a fence with about 1/8 acre of space and get anything I toss to them. I toss scratch every now and then if I don't have anything from the kitchen and they don't seem fat and sluggish and sometimes they don't even finish what I toss, but then come back later.
I think chickens are just programmed to wander and eat, and then hang out a preen and snooze, create drama etc. If they are confined, then eating is probably done just out of boredom, just like the rest of us!!!!