Green facades provide a biotope and shelter for small animals, promoting ecological diversity: they attract insects, butterflies, birds, etc... If you are not keen on spiders, mice (yes, they too...) and chirping sparrows coming too close to the windows, you can limit the greenery with cleverly arranged trellises.
A facade greening can serve birds and insects temporarily, but can also provide a permanent habitat. There is a real need of such refuges and biotopes today in our highly built-up urban environment! Some bird species (blackbirds for example), beetles, and other little critters like to nest in a wall garden. Spiders do not like to leave the greenery at all and enjoy eating other insect there: if the trellises and climbing plants are at a good distance from the windows, there is no reason for them to reach our homes. The correct placement of trellis systems 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 it wonderful scent.
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...