Regarding the origin: they don't actually have to retell the origin story, but I can understand why they'd want to. Retelling the origin gives an opportunity to build the character from the ground up.
The important thing is establishing who their
version of Superman is. Superman is a character who has been written a lot
of different ways. You can't just say, "Use the Superman everyone knows" because there are so many different versions of Superman. On a superficial level, there are a lot of shared similarities between different versions of Superman, but on a deep, personal level, what events have occured in this Superman's life define who he is at his core.
An origin story isn't necessary for that; Incredible Hulk was able to establish their version of their character pretty well while only touching very briefly on their version of his origin. But it does help a lot. An easy way to let the audience in and show them your version of the character is by letting them see the defining moments that shaped that character's life for themselves.
Because those are the moments that build the character, and shape him into someone that's interesting to see onscreen. I mean, you could say the same for any other iconic film character. Look at Batman Begins. I could tell you that everyone knows Batman's origin, so the entire first chunk of the film can just be cut out; the audience doesn't need to see Batman being trained under Ra's al Ghul, or learn about the Scarecrow fear toxin's history with both the League of Shadows and Bruce Wayne's history, or see the confrontation where Bruce gets thrown out in the streets by Falcone, because everyone knows Batman's history, right? All of that is stuff that happens to every Batman, right? So we can just skip over it?
The movie would have been lesser and probably nonsensical if we'd cut those scenes out, regardless of how well-known Batman is.
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