Some writers create these fanfics because it allows them to explore different character dynamics. When Voldemort cheats, it shows his desperation or his devious nature in a new light. And it also gives Harry a different kind of challenge to overcome, not just a physical or magical one but also a moral and strategic one. For example, if Voldemort cheats by bribing people around Harry, Harry has to decide how to deal with those who have been turned against him, whether to forgive or fight back.
One interesting fanfic could be where Voldemort cheats in a magical duel against Harry by using a forbidden spell that gives him an unfair advantage. In this kind of story, Harry might have to figure out a way to counter this cheating without breaking the rules himself.
Yes, there are quite a few. Some explore scenarios where Voldemort uses underhanded tactics against Harry in really creative ways. For example, in one fanfic, Voldemort manipulates the magical laws to his advantage during a duel with Harry. It makes for an exciting read as you see Harry trying to counter these unexpected cheats.
Some writers might write it for shock value. They want to create a scenario that is completely unexpected and different from the norm. By having Hermione cheat on Harry, it throws the familiar relationship dynamics out the window and grabs the reader's attention.
One interesting concept is when Voldemort cheats by infiltrating Harry's dreams. He can manipulate the dream world to make Harry doubt himself or give away secrets. It adds a psychological aspect to their conflict. Another is when he cheats by using ancient magic that no one expects. This often leads to Harry having to research and learn new magic quickly to counter it.
Some exist because some fans really ship Harry and Hermione (HHR) and feel that the Weasleys, especially Ron, are in the way of their ideal pairing. So they create fanfics to bash the Weasleys to make their preferred relationship seem more likely.
Some 'Harry Hurts Ginny' fanfics exist because it adds a new layer of drama to their relationship. Their relationship in the original series was mostly positive, but exploring the negative aspects like Harry hurting Ginny can make for an interesting and complex story.
For the sake of creating drama. Lies can easily stir up emotions and conflicts, which makes for an interesting plot. It keeps the readers on their toes, wondering when the truth will come out and how Harry will react.
Well, some authors might want to explore different character arcs. By having Ginny dump Harry, they can show Ginny as a strong - willed character who is not afraid to make tough decisions. It also adds drama to the story, as the relationship between Ginny and Harry is a central part of the Harry Potter universe.
Some exist because fans like to explore different power dynamics in relationships. They want to see how Harry, as a central and powerful character, can influence Ginny's actions and decisions.
It might be for the sake of exploring unique and unexpected interactions between the two characters. Their relationship in the original books is complex, and this is a very different way to add a new dynamic.