Fanboys like having their comic book characters and stories line up perfectly without any conflict. They are pretty dogmatic, and this can be a problem because their beloved characters are owned by corporations who keep using the same characters in many different books for decades. Different authors, illustrators, and editors can muck up the continuity of these stories. In order to try to make all of the stories fit together, they came up with the idea that these superheroes all live in parallel dimensions.
This multiverse got very confusing, and in the 1980s, DC Comics tried to simplify their reality and created the Crisis on Infinite Earths event. The event spread through many titles and had just about every DC character involved.
I don’t think they did their job properly though. They should have killed off all their characters and tried inventing something new instead of continually telling the same stories with the same characters over, and over, and over again.