Why is ventilation such a big deal?
Because chickens are amazing producers of moisture, ammonia and heat, that's why. Small but mighty! (Mighty messy anyhow).
1) Ventilation removes dampness and humidity from the coop. Chickens generate scary amounts of water vapor, partly through breathing out (same as we do, that's why a mirror fogs when you breathe on it), and largely through pooing (chickens do not urinate as such - all the water they would be peeing out if they were any other sort of animal is contained in their poo). They process a lot more water than you might think. All of this water tends to make the coop air humid. High relative humidity (especially in cooler temperatures) makes chickens more susceptible to respiratory disease and increases the chance of frostbite. Chickens can stand considerable cold without frostbite if the air is dry; not so much if the air is clammy.