Excessive moisture in the attic can cause mildew, musty odors and boards to swell. A common swelling-board symptom is a crack between the room wall and the ceiling. In the winter, the moisture can condense on the roof deck and drip onto the insulation. This can get bad enough to ruin insulation and stain the ceiling. Conveniently, both heat and humidity rise.
Hot air rises above cool air, and moist air is lighter than dry air.
All these holes allow the bad air out and good air in. Attics under other types of roofs need added ventilation. Having turbines, electric fans or other vents to exhaust hot, damp air will do nothing if other air cannot come in to replace it. Without soffit vents, preferably continuous soffit vents, the air will not flow efficiently and very little ventilation takes place. Properly installed, electric exhaust fans will not remove any more heat or humidity than an unpowered roof turbine, which in turn works no better than a hole in the roof.