The relationships in the original 'red white and royal blue' are so captivating, and fan fiction allows for more exploration of those relationships. For example, seeing how they handle different challenges in their relationship in new scenarios. Also, it gives fans the creative freedom to add their own twists and turns to the story, which is really exciting.
The world - building in the original work is quite interesting. Fan fiction often expands on that. It can take the political and social settings from the original and develop them further. Moreover, it gives fans who are really into the story a platform to express their own ideas and interpretations, and that's what makes it so appealing.