Fanfiction contributes by expanding the universe. With stories about Rayla and Callum, it attracts more people to the The Dragon Prince fandom. When fans read interesting fanfics about them, they might recommend it to others. Also, it can fill in the gaps between seasons. If there's a long wait for a new season, fanfiction can keep the characters fresh in people's minds. Writers can explore different 'what - if' scenarios, like what if Rayla and Callum had a big fight and how they would resolve it, which adds depth to the characters.
Fanfiction allows fans to explore different aspects of Rayla and Callum's relationship that may not be fully shown in the original series. It gives a chance to create new adventures for them, which in turn keeps the interest of the fans alive. For example, if a fan really wants to see them in a more romantic or action - packed scenario, they can write it in fanfiction.