Green facades provide animals with shelter, promoting ecological diversity: they attract insects, butterflies, birds, etc... If you are not keen on spiders, mice (yes, they too...) and sparrows coming too close to the window, you can limit this with wisely arranged trellises.
A facade greening can serve birds and insects temporarily, but can also serve as a complete wildlife habitat. The importance of such refuges is real today in our highly built-up urban environment! Some birds, such as blackbirds, like to build their nests in a wall garden, but also beetles and other creepy-crawlies can also settle here. Spiders, for example, like to stay in the habitat and prey on other insects. The correct placement of trellisis can keep creatures from the windows.
Wall gardens are also good for the protection of birds: insects in the garden serve as food for birds which like to look around and grab a snack! They will often even build their nests in the trellis! But many climbing plants themselves are food for animals-- ivy berries are popular with birds, while those of firethorn become winter food. Virginia creeper and ivy serve as a bee meadow, and blooming honeysuckle attracts night lovers (nocturnal insects, etc..) with their beguiling smell.
Some mice and raccoons can climb greened walls! If fruits are on a trellis, wasps and bees can be troublesome. Also sparrows in the ivy can be disruptive... Ahhhh, nature...