A standard roof that comes together to form a peak needs a ridge vent to provide adequate air flow so that the building does not trap moisture. Shed-style roofs do not need ridge vents. They rely on vents on each eave for ventilation.
A ridge vent is essential to making sure a standard peaked roof allows a building to vent properly. Without proper venting, a building can trap excess moisture, which can lead to mold, mildew and decay.
Buildings typically have a minimum number of eave vents as well. Shed-style roofs only have eave vents, but because the roof does not join together to form a peak, air can easily flow from one eave to another.