Heat and moisture are hard on roof shingles and roof sheathing. In the hot summer months, a poorly ventilated attic will dramatically reduce shingle life and may even void the shingle warranty.
In the colder winter months, moisture from everyday activities inside the house can enter the attic. Without proper ventilation, this moisture promotes mold and mildew, saturates insulation, and can freeze on the underside of the sheathing. In the summer, heat from the attic can radiate down into your living area making the air conditioner work harder to cool your home.