Attics need an appropriate amount of airflow and insulation. Balance is needed. The airflow occurs from the soffits shown in the picture on the left where air flows up and it can exit at the vents at the peak. The warm moist air that rises from inside your home in the winter can get trapped in your attic if you do not have good air flow. This is made even worse if you do not have adequate insulation levels. This can cause moisture to condensate underneath the plastic vapour barrier above your drywalled ceiling and cause moisture damage in your home. Often times people think this is due to a roof leak but it's all in having the right air flow and insulation levels in your attic.