You can't get much more foreign than being from another planet. (Maybe being from a different dimension would be more foreign? We're not sure.) When the War of the Worlds Martians first appear out of their cylinder, the narrator wants us to know just how terribly different they are, what with their tentacles and head-bodies and lipless mouths. But at the same time, the narrator also makes several connections between them and us. For example, both humans and Martians have been known to invade and destroy. If this book were an after-school special with a happy ending, the lesson might be something like this: "We're not so different from Martians after all – we're both jerks. Let's go get a coffee and talk about how to kill people."
The Martians have to be depicted as utterly foreign because Wells is trying to tell us a realistic, science-based story. However, the Martians also have to be similar to the British in order for the moral to get through.
In Wells' War of the Worlds, we learn that foreignness and difference can always be overcome by finding some similarity or connection.