Superman, Supergirl, the Martian Manhunter, Silver Surfer, Jemm Son of Saturn, the New Gods and Forever People, Samaritan, Captain Mar-Vell, Hawkman and Hawkwoman, Zauriel, Miss Martian, Thor, Starfire, Wonder Woman, Namor, Rom Spaceknight, the Transformers, Omni-Man, Icon, many other Superman analogs, and in the realm of comedy, Ford Prefect, My Favorite Martian, Alf and 3rd Rock from the Sun... What they have in common is that they are aliens among us, most of them publicly so. As you can see from the list, we can even count heroes from Earth who come from places that might as well be alien worlds, like Amazon Island, or that are other dimensions, like Asgard. Even characters from the past or future stranded in the past - say the Legion (Lost) or the Shining Knight could be considered "strange visitors".
These are not merely "alien" heroes. They are the alien confronted with humanity. Their powers (and often, attitudes) set them apart, but as they learn more about us, they BECOME more like us. As we, in return, can see ourselves through the alien's point of view. And just as the hero is alien to us, we are alien to him, her or it. Will we embrace this savior from another world, or will we xenophobically reject the unlike? That's on us. Superman remains a special case because, Tarzan-like, he was raised on Earth, by human parents. He's physically an alien, but he has very human values and attitudes. So our reaction to him is a more important piece of the story (ergo, all those essays on what Superman means, and one of the more iconic human supporting casts in comics history). But there's something else these heroes bring to the table, whether they "act human" or soliloquize on how strange we are (looking at you, Surfer!). Can you guess what it is?
Each of those characters comes with a strange and interesting cultural background, a whole world to explore on the comics page. Flashbacks, chronicles, contemporary visits, each appearance and reference adds to the world and thus, to the hero. And because these places weren't home to a single inhabitant, they tend to spawn new members of the hero's family, friends as well as enemies. Krypton may be gone, but Superman is far from the Last Son of Krypton he once was. We're interested in Strange Visitors because they are our window into the worlds from which they came. If it's all an immigration story, it's about embracing what the immigrant brings to one's own country.
Why do YOU like these visitors to our shores? Do you revel in the minutiae of their alien worlds, or would you rather explore what it means to be human with them?