Attics need ventilation for several reasons, including energy efficiency of the home, ice dam prevention, moisture control and shingle protection. Manufacturer warranties on asphalt shingles often requires ventilation.
Vents and attic fans are two methods of adding ventilation to the attic. Vents allow air to flow freely into the attic area. Attic fans work with the vents by drawing outside air through them and moving hot air out of the attic. Fans require proper sealing between the home and the attic to prevent the house air from being pulled out by the fan.
Attic ventilation helps the home in both winter and summer. In cold climates, a warm attic in the winter causes snow to melt, but once it reaches the gutters, it refreezes. The ice builds up over time and develops dams that damage the roof. If the home has proper insulation and is sealed well, the cold air in the attic doesn't affect the heating system.
In summer, attic ventilation also helps cool the air. A hot attic can cause damage to shingles. The ventilation also keeps the air moving during all seasons so moisture doesn't build up in the attic space. This can prevent rotting of wood in the attic.