Green walls in urban areas reduce temperatures of buildings they grow on. The primary cause of heat build-up in cities is the absorption of solar radiation by roads and buildings in the city, the storage of this heat in building material, and its subsequent re-radiation. Plant surfaces do not rise more than 4–5 °C above the ambient and are sometimes cooler.
Green walls may also be a means for water reuse. The plants may purify slightly polluted water by absorbing the dissolved nutrients. Bacteria mineralize the organic components to make them available to the plants.
They look beautiful, cut down on warming, and recycle water. Everone wins!