In fact, if you ever hear rodents running around in your attic or if your ceiling is cracked and yellow... chances are you have a problem with attic ventilation. Also, if you have high utility bills in both the summer and winter you could have a problem with attic ventilation.
The roof just makes the house water tight. The space between the insulation and the roof is of course the attic and it needs to be ventillated to prevent heat build up in the summer, (up to 140-degrees) and moisture build up in the winter. That's why roof and soffet vents are installed... to keep the air circulating and prevent the build up of heat and moisture.
Proper ventillation starts with fresh air being drawn in through the soffitt vents which you will find underneath the eafes of your house. Make sure the vent openings on the soffits have not been inadvertently covered with insulation... this happens a lot during new home construction.
The roof vents are what pull the air up through the attic...
There are several different types of roof vents:
Ridge and gravity vents: these are standard installation that just sit on top of your roof... there's no fan involved... but it does allow the hot air that's rising in your attic to escape... so it does move some air but not a lot.
Wind turbines: now these are an upgrade, they are easy to install if you have an existing home without them, when the wind hits the turbine it spins and creates an outward draught in your attic. Also, if there is enough heat build up in your attic this will cause the turbine to spin as well and through the process of inertia causes air to move through your attic... a heck of a lot more than a simple gravity vent.
Solar and electrical vents: these are a definite upgrade but may be worth it. The solar vent runs the turbine using a solar panel for energy so there's no electric bill involved. The electric vent of course uses electricity so it will add to your monthly bill. But these systems keep the air circulating in your attic at all times. Therefore reducing heat, mildew and other problems.