Longer Growing Season - Temperatures in a greenhouse do not vary as much because the radiation of the sun is trapped in the enclosure, keeping the heat in the structure.
Garden in Any Weather - Since everything is covered, you don't have to worry about getting wet. Even if the rain is pouring outside, you can keep your garden dry.
Grow a Wide Variety of Plants - Experiment with exotic plants not found in the local area.
Protection from Small Animals and Predators - Helps prevent small animals and predators like moles, deer, and squirrels from eating your harvest.
Keep Beneficial Insects Inside - Some insects, such as ladybugs, are beneficial to plants. Beneficial insects can not leave easily in a greenhouse.