Good ventilation is very important. Having a roof over your head is important for many reasons, but the two main functions your roof provides are 1) good ventilation and 2) redirecting water without good ventilation. Hot and moist air builds in your attic space and between roof shingles and plywood. Without proper ventilation, this is a haven for algae growth, mould growth, and dry rot. The soffit (the material beneath the eave that connects the far edge of your roof to the exterior wall of your house) is where all the hot moist air blows out, to the top of the roof, through the vents. Basically, good ventilation allows your roof to stand the test of time.