Another reason is to give Harry a fresh start. In the original series, Harry has been through so much. By making Harry female and sending her out of the wizarding world, it allows the character to have a new beginning, free from the past traumas and expectations. This can open up a whole new world of possibilities for character development and story exploration.
It could also be a way to comment on the wizarding world itself. Maybe the author feels that there are certain aspects of the wizarding world that are not so great. So, by having fem Harry leave, they can show the problems or limitations of that world. For example, if fem Harry leaves because of discrimination or political unrest in the wizarding world, it can be a statement about those issues.
Some authors write it because they want to explore different character arcs. Harry's character is so iconic in the original series. By making Harry female and having her leave the wizarding world, they can create new and unique storylines that deviate from the traditional narrative.
There's also 'Farewell to Magic' in which fem Harry leaves after a big magical war. She realizes that the wizarding world has changed too much and she can no longer find her place there. So she travels far away to a place where magic is just a faint memory and builds a new life from scratch. She has to learn how to adapt to a completely non - magical environment which is full of challenges but also new opportunities.
Authors write such fanfictions because it gives them a chance to explore Harry's character development further. By having him leave to train, they can show his growth outside the familiar setting of Hogwarts.
Some might be shocked and feel betrayed. Harry is a hero to many, so his departure could leave them feeling abandoned. Others might understand his need for a break and respect his decision.
Another reason could be to give Percy a chance to find himself. In the original stories, he is often defined by his role as a demigod and his relationships. When he leaves, he can start to figure out who he is without those labels, and authors can use this to delve deeper into his personality and motives.
Maybe he thinks it distorts the real story of the wizarding world he knows. Fanfictions often take creative liberties that might go against his own experiences and the original narrative.
It could be that Harry has faced so many challenges in the fanfiction that he's simply burnt out. Just like in real life, if a person endures too much stress or hardship, they might want to step away. In the fanfiction, Harry may have reached his limit of what he can handle in the wizarding world.
In a fanfic, Hermione might initially be in denial. She would keep expecting Harry to return any day. She would go to their old haunts in Hogwarts, like the library or the Gryffindor common room, and just sit there, reminiscing. As time passes, she realizes she has to move on. She starts teaching at Hogwarts and imparts her knowledge and the values Harry also held dear to the new generation of students.
Some write it to explore alternative storylines. Maybe they thought the original plot could have taken a different turn if Harry left at a certain point.
Some might be in shock. After all, Harry has been such an important figure in the wizarding world for so long. His friends like Hermione and Ron would probably be very worried about him and try to find out where he went. The Ministry of Magic could be in a panic, fearing that without Harry, they'll be vulnerable to new threats.
Well, it could cause quite a stir in the wizarding world. First, the relationship between Harry and Draco has always been complex and full of rivalry. If Harry leaves Draco for Ginny, the Malfoy family might feel a sense of victory in a strange way, as Draco has been in this sort of unspoken competition with Harry. For Ginny, it would be a big step forward in her relationship with Harry. And among their friends, like Hermione and Ron, they might have mixed feelings. Hermione might be worried about how this would change the dynamics among them all, while Ron would probably be more focused on how it affects Harry's happiness.