They form the foundation. The origin stories in graphic novels define who Superman is at his core. For example, they explain his powers, his moral code, and his connection to Earth and Krypton. This understanding then shapes all the movies, TV shows, and other media related to the franchise.
Often it starts with mutual respect. They recognize each other's abilities and dedication to justice. For instance, Superman respects Batman's ability to fight crime without powers, and Batman respects Superman's selfless use of his powers.
Superman's character might change in that he starts to question his place in the world more. The relationship with Wonder Woman makes him think about a future that involves not just saving the world alone but with her by his side. Wonder Woman could change in her perception of power. Instead of just relying on her own physical strength and Amazonian heritage, she could start to see the power of love and partnership with Superman. Their relationship in the lemon fanfiction could be a catalyst for these internal changes in both characters as they navigate this new aspect of their lives together.
Some characters might be critical. If they have a strong sense of the 'canon' Superman, they could nitpick at the details in the fanfic that don't match up. For instance, if the fanfic changes Superman's origin story too much, they might frown upon it.
It depends on your level of interest. If you want a deeper understanding of Superman's origin and early adventures, Action Comics can be a great read. But if you're just looking for a basic acquaintance, there are other options available.