Self - discovery can be a theme as well. The reader, through their experiences with the AoT girls, may learn more about themselves, their strengths, and their weaknesses. It's a way to explore the character's growth within the unique context of the Attack on Titan universe.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
There's also 'With the AoT Girls'. This fanfic focuses on the daily lives of the AoT girls and how the reader becomes a part of it. It shows their relationships in a more relaxed and personal setting, which is really endearing and gives a different perspective on the AoT world.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
It could mean that a person who is a fan of 'Attack on Titan' (AOT) is reading or watching fan - created fictional works related to AOT. Maybe they are interested in seeing different interpretations or storylines that fans have come up with.