How does putting my plants in a greenhouse help them grow? Is it purely to shield them from cold weather, or are there other benefits?
There are several benefits of a greenhouse:
accumulation of heat from the sun
possibility to regulate temperature and humidity
Protection from wind and excessive rain
protection from unwelcome pollens
limited protection from pests especially birds.
Typically you put in a greenhouse plants that prefer a warmer climate than the local one.
Primarily, greenhouses trap heat and moisture.
Most plants prefer things warm and damp, so it's an ideal environment for growing fruit and germinating seeds.
Green houses can extend your growing season, and even allow you to grow throughout the year.