It could slow down the plot. For instance, if Harry's glasses are constantly malfunctioning, he might miss important clues or signals. This would add more obstacles to his journey of solving mysteries or fighting against the dark forces.
In this fanfiction, his glasses could have a disorder where they constantly fog up, no matter the temperature or humidity. It would be really annoying for Harry. He might be in the middle of a battle with Death Eaters or trying to solve a mystery in Hogwarts, and he has to keep wiping his glasses to see clearly.
Your glasses in the Harry Potter Eclipse fanfiction could have been affected by a powerful spell, or perhaps you left them somewhere and forgot. The story's imagination determines exactly what happened!
In some fanfictions, the glasses could be a key to a hidden realm that is only accessible during an eclipse. Harry discovers this by accident and has to enter this realm with his friends. There, they face challenges that are related to the very essence of magic and the origin of his glasses.
A possible plotline could be that during an eclipse, Harry's glasses become a portal to another dimension within the wizarding world. He steps through and finds himself in a place where the rules of magic are different. Another plotline might involve the glasses being enchanted by an eclipse - influenced spell that causes Harry to see things that others can't, like hidden messages on Hogwarts' walls.
Perhaps there's a plot against Harry by some dark forces, and the Potters believe that only they can protect him properly. They might think that the people who have been taking care of Harry so far are not equipped to handle the new and more dangerous threats. Also, it could be that the afterlife in the wizarding world has some rules or opportunities that allow them to try and get Harry back into their family fold. They could have been given a second chance, so to speak, and are eager to take it.
One possible motive could be that the Potters discovered some dark plot by the Malfoys involving Harry. Maybe they found out that the Malfoys were planning to indoctrinate Harry into their pure - blood supremacist ideology against his will. So, to protect Harry from that, they kidnapped him.
Yes, they might. Just like any other group of people, those who identify as Blacks or are part of the Potter family name (if we assume there are real people with that name) could be fans of the Harry Potter series and thus read fanfiction. Fanfiction offers a wide range of interpretations and extensions of the original story, and anyone with an interest in the magical world of Harry Potter could be drawn to it.
Sirius Black could also be part of it. He was Harry's godfather. He might have escaped from Azkaban earlier in this fanfic. He teams up with the Potters to rescue Harry. He could be the one who scouts out the Dursleys' house before the actual rescue operation, using his skills as an Animagus to stay hidden.