Your rooftop needs to breathe. Your home has to endure a lot of drastic temperature shifts and weather conditions throughout the year. It's your home, afterall.
As the years weather upon your home you may find yourself with a myriad of issues, but ventilation is easily overlooked as the cause.
Simply put, without proper ventilation, your roof will not release heat properly during the winter time, hence it will cook a layer of snow, which runs downward, and refreezes where the heat is not as intense.
As the problem worsens, you will have a first weakened, then outright sagging portion of rooftop, and if left unchecked, it will eventually come through (this will take several seasons.. it will not happen in one).
In short : It is important to ensure that your roof is properly ventilated.