Proper attic ventilation will allow a continual flow of outside air through the attic, protecting the insulation in the attic, which helps to lower temperatures in your home, It also helps to keep your roof from premature failure and is required by all shingle manufacturers for a valid warranty.
You must have the correct balance between the air intake (soffit vents), and the air exhaust system (ridge vent, power vent, static vents) which is at or near the highest point of your roof.
In the winter months when the attic is much cooler, the heat from running your heater along with various household appliances, showers, etc…can contribute to excess moisture build-up in the attic. If the attic is not properly ventilated the moisture will condense and collect in the attic insulation and reduce its efficiency also known as R-value. This can lead to mildew growth and buckling of felt and shingles.
To summarize, a properly ventilated attic and roof can help reduce the load on your air conditioner by moving the hot air out of your attic before it builds up and causes damage to your roof and it prevents moisture build up which can lead to mold, mildew and big decrease in your attic insulation efficiency.
Although we do not run into this problem in the south very often, proper attic ventilation also prevents ice dams which can lead to costly roof leaks. Usually not a problem here in the south, but every now and then we do get a bad winter storm which can lead to ice dams on the roof.
Make sure your Hartwell roofing contractor understands how to properly configure your air intake and exhaust as recommended by the FHA. This will help ensure your new roof lasts like it is supposed to and you have a legitimate warranty from your shingle manufacturer.