It allows for unique character interactions. Imagine a character from Fallout meeting a character from The Witcher. Their different values, skills, and goals would lead to some really engaging dialogues and storylines. The contrast between a post - apocalyptic survivor and a monster hunter can be very entertaining. Also, it gives writers a chance to explore new themes. They can take the bleakness of Fallout and mix it with the hope or despair of another universe, which can result in some deep and thought - provoking stories that are over 100 words. For instance, in a 'Fallout x Dark Souls' crossover, the themes of death and perseverance from Dark Souls can be intertwined with Fallout's survival aspects, creating a story that makes readers think about the meaning of life in a desolate world.
It's interesting because it lets fans of both franchises see their favorite elements mashed up. If you love Fallout's Vaults and also like the cybernetic enhancements in Deus Ex, a crossover fanfic can bring those two together. You can see how the Vault - Dweller might react to the advanced augmentation technology, and it gives a fresh perspective on both universes.