A greenhouse is a contained quantity of air and light which extends the growing season and enables the thriving of plants otherwise alien to local climate.
It creates a bubble of super-nature, where things otherwise impossible become possible. The smell, temperature, humidity and taste of things are significantly different than just outside the thin membrane.
It enables us to create different kinds (or quantities) of food, but also to experience different climates and atmospheres.
A greenhouse is by definition alien to its site, but creates a significant place in everyday life.