Ever since I read 'Project Hail Mary' and how the whole plot spins around Rocky, I've realized aliens aren't just set dressing. They force the narrative to deal with things like first contact protocols, translation puzzles, and fundamentally different biologies that can create conflict out of thin air. A human crew might be fine until they meet a species that communicates through scent or sees time in reverse; suddenly the adventure isn't about navigating nebulae, it's about navigating a total breakdown in understanding.
Some stories use them as a pure obstacle, like in 'Alien', where the creature is a relentless force of nature. Others, like in Becky Chambers' work, use alien perspectives to question what 'adventure' even means—challenging the very human-centric idea of conquest and exploration. The alien influence shifts the story's moral compass, turning a straightforward mission into a meditation on coexistence.