In many fanfictions, the idea of Bill Weasley flirting with Hermione can add an interesting dynamic. It might be a way to explore different character relationships outside of the main canon. Some fanfic writers may use this scenario to develop Hermione's character further, showing how she reacts to the advances of an older, more worldly wizard like Bill.
Fanfiction has a vast scope for exploring different character interactions. When it comes to Bill Weasley flirting with Hermione, it could stem from the fact that both are intelligent and have their own areas of expertise. Bill, with his knowledge of the wizarding world beyond Hogwarts, and Hermione, with her thirst for knowledge. This flirting could be a way to bridge the gap between their different worlds within the fanfiction. It might also be a way to introduce a love triangle - like situation if the writer wants to add more drama to the story.