Perhaps Harry has a plan to outwit Voldemort in the long run. By seeking help, he can get closer to Voldemort, understand his weaknesses better, and then use that knowledge against him. It's a risky but potentially effective strategy, like a spy infiltrating the enemy's camp.
Perhaps Harry discovers some dark secret about his own past or powers that he thinks only Voldemort can help him understand. For example, if he finds out that his connection to Voldemort is more complex than he thought and that Voldemort holds the key to unlocking or controlling certain aspects of his own magic.
Well, in many fanfictions, Harry might use his knowledge of hidden places in the wizarding world like the Room of Requirement. He could also rely on the help of his friends. For example, Hermione might use her vast knowledge of spells to create a hiding place or a protective charm. And Ron could be there to offer physical protection if needed.
Perhaps in this fanfic, there has been some major plot twist that makes Harry believe that bowing to Voldemort is the only way to save his friends or the wizarding world. For example, Voldemort may have threatened to kill all of Harry's loved ones if he doesn't submit, and Harry is sacrificing his pride for the greater good.
One reason could be a desire for revenge. Maybe in the fanfiction, Voldemort has done something even more heinous to Harry's family or friends, driving Harry to take extreme measures.
The wizarding world might be in shock. People who have always seen Voldemort as the ultimate evil would be confused. They would have to re - evaluate their views on Voldemort. Harry, on the other hand, would be in a very strange situation. He might be distrusted by some of his friends who think he has made a deal with Voldemort.
Perhaps Voldemort discovers that there is a greater threat out there. In the fanfic, it could be that another dark force is emerging, and Voldemort sees Harry as an ally of sorts against this new threat. So, he decides to protect Harry in order to deal with this common enemy.
Some people might create such fanfiction for the shock value. It's a very different and unexpected take on the characters. They want to see how they can twist the traditional story.
Harry could put himself in great physical danger. Voldemort might not be willing to help without exacting a high price, like Harry's own life or his magic. And even if Voldemort does seem to offer help, it could be a trap to harm Harry.
Themes could include the blurred lines between good and evil. If Harry is a fan of Voldemort, it challenges the traditional view of their roles. Another theme might be the exploration of dark magic. Since Voldemort is associated with it, Harry's fascination could lead to a deeper look into the nature of dark magic.
In some fanfiction, Harry leaves to seek more powerful magic. He realizes that the magic in Britain is somewhat limited in the fight against Voldemort. So he travels to other places, like ancient magic - rich regions in Asia or Africa, to find a way to defeat Voldemort. He hopes that by leaving, he can gain the upper hand in the battle.