If vents are added correctly which means having an equal amount of ventilation at the highest point of the roof(peak) and the lowest point of the roof(soffit). This creates what is called the "chimney effect" as hot air rises and exits the vents at the peak of the roof it pulls cooler air through the soffits, cooling the attic and removing moisture.
To figure proper amount of ventilation.
Minimum-1 sq.ft. of ventilation (NFA) for every 300 sq.ft. of attic space. 50% of this number goes to the peak and 50% to the soffit. I like to use a ratio of 1/150.