Following early screenings of Batman V Superman, some critics looking for a reason to attack the movie have taken issue with Wonder Woman in the movie. Despite her appearance being the highlight of the NYC premiere for many, some of these critics have attacked her appearance in the movie as making no sense and that she has no story in the movie.
That’s because some people don’t really pay attention in movies and miss key details.
Diana Prince shows up in the movie because she’s after something Lex Luthor has in his possession. As Bruce Wayne is going after the Kryptonite that Lex has, Diana (who is an antiquities collector like all movie immortals) wants to steal back a photo of her that Luthor is keeping. This isn’t just any photo, it’s a picture of her from 1918 and would expose the fact that Wonder Woman is immortal. It’s one of many pieces of evidence of metahumans that Lex Luthor has.
That’s what brings her into the story, and why Wonder Woman is there to fight Doomsday with Batman and Superman. It’s also part of the Justice League setup as Luthor’s metahuman evidence is the way they give cameos to The Flash, Aquaman, and Cyborg.
So when someone tells you that Wonder Woman shows up for no reason in the movie and she’s just tacked on to the story, they really didn’t pay good enough attention to it.