A usual city is consist from building, machines and people. They all seems to be a part of the bigger lifeless structure or mechanism. But Banksy's animals are dynamic and out of place. They looks like the only living beings in the city. Or rather they hint how it could be, because they are all just pictures and there is no place for real animals. But we often could see something like this in movies, so Banksy's animals touch this theme.
People try to find author's meaning in each graffiti, but fail to see the whole thing, IMO.