What's the point of the aliens in Sci-Fi?

Klingon’s, Vulcan’s, Hutt's, Navi. Space opera just wouldn't be space opera without a few intelligent aliens around. Why do storytellers feel the need to include them? Is their a function that the alien serves that humans would not? Are they just there to highlight the strange setting of the story? It really varies from story to story and sometimes from species to species.
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