Proper roof ventilation is important to properly manage the heat in your attic, prevent condensation, and halt the early aging of roof materials. In general, if you have a proper roof ventilation system (which includes both exhaust and intake vents) the heat in your attic will be reduced in the summer and the moisture in your attic will be reduced in the winter. This makes your heating and cooling bills more efficient but also extends the life of your roof. If you do not have proper ventilation, you risk excessive heat in your attic, mold growth, frost in your attic, or excess water condensation in your attic (this may appear to you as a leak).
A proper ventilation system must include a way for air to escape the attic, but also a way for new, fresh, cooler air to enter in. As such, this system includes both exhaust vents (such as turtle vents or ridge vents) as well as intake vents. Intake vents are typically installed under the overhangs of your roofs, called the soffit, because this area is shaded and thus, cooler air is located here.
Exhaust vents may be installed as either turtle vents (which look like little boxes installed close to the top of roof) or a ridge vent (which is virtually unnoticeable as it is installed right on the ridge of the roof). While turtle vents are relatively inexpensive and easy to install, they are not nearly as efficient or as aesthetically appealing as ridge vents, and they are more prone to leaks. Ridge vents, on the other hand, allow for a more continuous flow of air and are more dependable (i.e., less likely to leak).
Regardless of which type of ventilation you choose, make sure you work closely with your contractor to ensure that there will be enough airflow both in and out of your roof. Proper ventilation will save you money on energy costs and will ensure that your roof performs at its utmost ability.