One of the foremost exponents of vertical gardening, Patrick Blanc, features in a CNN article about green walls that states:
Designers claim that plants not only provide a beautiful alternative to other materials used in construction, they also offer additional benefits, including noise reduction, natural cooling, air conditioning and a psychological boost to those who use the buildings.
Depending on the type of planting used, and where the wall is sited, there are habitat benefits to be had for the local wildlife. Some designs purposely incorporate bee boxes and nesting sites for birds.
Compared with other approaches to greening the urban environment: green roofs, parks and planting trees, vertical gardening is admittedly very high maintenance, with water and light being the main issues.
Some of these walls are actually sited indoors, or in internally facing courtyards, with poor quality light. The usual methods of keeping plants alive in these situations are plant selection, or artificial lighting: a poor environmental choice.
The watering problem is not an insurmountable one. Designs use drought tolerant planting at the top of the wall, with those preferring a wetter environment further down, an efficient watering system and water storage; rain water and even air conditioning condensate are used to reduce water costs.