A good ventilation fan is essential during all stages of a plants life. It evacuates hot, humid air from the garden and allows cool,dry air to enter. This makes it easy to keep temperature and humidity at proper level at all times. A garden without ventilation will become too hot and humid pretty fast which affects growth.
Plants also use up all available carbon dioxide in a room within hours so new co2 rich air must be introduced into the garden at a regular basis. Co2 is crucial for photosynthesis and overall health of plants. If you are only growing a few plants you may get away without a fan by opening a window or door nearby. Larger gardens should always have fans to ensure rapid growth and to avoid problems