Maybe he wants to keep up with the latest news in the wizarding world. There could be important announcements, like news about Voldemort's activities or updates on the Ministry of Magic.
It could be that he's looking for information about his friends or Hogwarts. For example, if there are any articles about Quidditch matches at Hogwarts or news about Hermione's latest achievements in the field of magic study.
He might be interested in the articles about new magical creatures. Since he has had many encounters with different and often dangerous creatures, news about newly discovered ones or updates on known ones would surely catch his attention.
It could also be a strategic move. Gringotts is a highly secure place. By buying it, Harry could ensure the safety of important magical artifacts or secrets. He could use it as a base to store and protect things that are crucial for the future of the wizarding world, like ancient spells or prophecies.
One reason could be defamation. If The Daily Prophet in the fanfiction spreads false and harmful rumors about Harry Potter, that might lead to a lawsuit. For example, if they wrongly accuse him of dark magic practices.
In this fanfiction, Harry Potter might be facing a new situation in the Muggle world. Buying a car could be a sign of his further integration into Muggle society. Maybe he uses his wizard money to buy a really cool and unique car, like a classic convertible.
One possible reason could be his past experiences with some cruel adults, like the Dursleys. They treated him horribly, so in fanfiction, this trauma might make him generally fearful of adults. Another reason could be his experiences with certain authority figures at Hogwarts who may have been strict or unfair, which could be exaggerated in fanfiction to create a fear of adults.
Ron might be there too. He could use his skills in handling magical creatures to populate the island with friendly ones. And he could also help with fishing in the surrounding waters using magic, providing food for those on the island.
One reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.