A closed terrarium protects plants from dust, pollution, and any dramatic changes in temperature or pressure while allowing in enough light for the plants to photosynthesis as normal. In a well-designed closed terrarium, the plants are self-sustaining, as the glass stops any of the humidity created from escaping.
The best terrariums are mini-ecosystems specially designed to illustrate how a natural environment works. Inside a closed terrarium's walls, many different biological, chemical, and physical processes can be observed, including photosynthesis, respiration, and the water cycle. The water in a terrarium is continuously recycled, passing from liquid form to gas and back again. As the moisture in the terrarium condenses on the glass walls, it returns to the soil and is absorbed by the plant's roots.
Closed terrariums can remain closed for years and years. There is not much maintenance required from your side.