Excess moisture has tremendous potential for damaging bathrooms. During a bath or shower, humidity levels rise significantly creating the perfect breeding ground for mold, mildew and microorganisms that can negatively impact health. In addition, long-term exposure to excess moisture and humidity can crack and peel paint and wallpaper, ruin wallboard, warp doors and rust cabinets and fixtures. Without control, it can even cause deterioration of joists and framing above the bathroom.
Most of today's new homes are highly insulated and practically air tight, saving energy but making proper mechanical ventilation essential to maintaining good indoor air quality. Good ventilation protects both your health and your home and is especially important in bathrooms which experience high levels of moisture and humidity.
-Source: Home Ventilating Institute