If you shower, cook, wash dishes, clean, do laundry – if you breathe – inside your home, you’re generating water vapor. Lots of it. Gallons, in fact. Every day. And if it’s not ventilated properly out of the attic, that water vapor can become a moisture problem in the winter that may not be evident during warmer weather.
https://www.roofingcontractor.com/articles/88605-attic-ventilation-is-not-just-a-summertime-concern