In summer, the primary purpose of ventilation is to expel hot air from the attic or roof to reduce the strain on air-conditioning systems. In the winter, the primary purpose of ventilation is to maintain a cold roof temperature to avoid ice dams created by melting snow and to vent any moisture that moves from the conditioned living space to the attic.
Many newer homes are built with ridge vents, an generally they do a good job. Here in the Midwest however, we see problems. In the winter, attic moisture can rise to the ridge and settle on ridge screening. As temperatures drop, the moisture freezes blocking attic air exhaust, thus trapping heat and moisture in the attic. The heated air melts snow on the roof deck, the melted snow runs down the roof deck to cold gutter where it freezes. Eventually the gutter fill, the ice pushes up under and around nearby shingles resulting in ice dams. Unaddressed, ice dams can cause damage to your roof structure, shingles and be costly to repair.
