I would have said it's a general indicator of strong immunity and lack of genetic faults. Also the sense to get in out of the way from a predator or whatever. I'm also separating hens who insist on not roosting and they won't be bred from. One hen who is determined to fill a nest box up with poo every night can ruin a whole load of eggs the next day.
I've had a fair few hens fall over and die at first moult or when it's gets hot or cold. Usually a heart problem or something. It's possible that I could have already hatched from that bird and that's what I want to avoid. If they make it to 3 with no issues and are still laying well shaped eggs with good shells, then I figure they are good breeding stock.
Plan is to hatch new birds around March and raise them up, selling any eggs cheap as pullet eggs.
1st Birthday the eggs start going on the gate as they should be nicely sized, maybe sooner.
2nd Birthday the best go in the breeding pens to produce high value hatching eggs.
3rd Birthday any good survivors are used to produce replacement hens and off we go again.
This is just a theory, I haven't got that far as yet!