I've had similar thoughts regarding superhero movies, whenever there's a reboot or whatever, there's always this instance on portraying the origin story. (Actually, even in comics.) I've never found that necessary really, although Batman Begins and Iron Man did some fun stuff with it. On the other hand all the flashbacks were the worst part of Man of Steel.
I guess really any established franchise, like the Star Trek movies. There's a cultural knowledge of the characters, do we really need to know where they came from every time? (Stuff like Harry Potter or Divergent or whatever is somewhat different since they're still making the franchise as the movies are happening.)
It's one thing I like about the way the Batman v Superman court drama film seems to be going I suppose, it would be cool to have the mystery and urban legend and so on stuff attached to the Batman character (and Wonder Woman) like they've existed already, we just didn't see them because we were watching the guy who can fly and shoot lasers out of his eyes and leveled a whole city and a bunch of Kansas, instead of laying out their life story for me. Same with Trek, he's Kirk, he's Spock, what more do we need to know?
I actually think it doesn't help new audiences and is more of a turn off. None of us fans came to Batman or Star Trek or whatever the fuck and started with their ORIGIN STORIES unless it was intentional.