Humidity is important in a HEATED air environment.
Hens are not managing humidity, they heat the eggs with direct heat that does not rob an egg of it's moisture.
In a box with a light or heating element you are constantly robbing the air and egg of moisture, so you have to do something about it or it dries out too much.
In the natural environment the egg loses it's 12% and rarely too much.
In the artificial environment of an incubator you can cook off 20% of the eggs moisture pretty easily if you do nothing to manage humidity. Then the wee buggers are pretty stuck. Too much, opposite problem, you can drown them.
That's why there are fairly set rules for artificial incubation. Natural incubation is a whole nother boat.