Comment by mapt

It happens a lot in efficient houses that don't cover all bases with HVAC (the vast majority of recently built houses), where the room door is closed, maybe the vents are not ideal, and there is usually no makeup air or forced air ventilation other than a furnace intake.

This change in scientific literature actually causes a ~quadrupling of recommended airflow ratios for tight homes versus ASHRAE's previous guidelines, putting strong emphasis on an ERV. Previously, ventilation needs tended to be dominated by air quality and smell, by humidity buildup, or by theoretical house parties that maxed out the system.

This ventilation adds capital expense, but it's substantially more controllable and significantly cheaper in the long run in colder climates than 'just open a window' or 'just don't build the house so tightly sealed'. Reserve the operable window for the aforementioned house party, which is out of a reasonable design envelope.