I know in the last piece I discussed mostly how backstory is used to establish character, and since thought of a couple more points I wanted to cover.
One - backstory also is used to establish setting. This is one of the biggest reasons why the opener of The Incredibles works, but Up doesn't. Where Up starts as a quiet bit that builds to adventure, the short movie that takes over the opening is used to tell us about the character, but nothing that couldn't have been done by stretching out his departure a bit. The Incredibles is an action movie. For us to know the characters, especially at somewhere near their best, before we see them at bottom, we need to see them acting. We need to see what they can be. Starting out with Mr. Incredible beaten down would make for a lousy start. You could imply the fall, but the contrast is far more effective, and starting with them down and out makes for a depressing opener.
Two - just rewatched Guardians of the Galaxy. Good film. Love the funky vibe, the mix of magic and science, the characters.
You could ditch the entire opener on Earth and it would not only not hurt the film one bit (one tiny exception), but would make discovering why he plays the music he does, why the walkman is so important to him, the earth references that are out of place, far more intriguing as we discover who he is. That opener is redundant as hell. One minor dialogue tweak could make it very clear in a vulnerable moment he lost his mom instead of implying it. Maybe, maybe add one line in that scene - where he's explaining why the walkman is so important - that he never knew his dad. The story makes it clear he was kidnapped - and not delivered as intended. That his father is not of earth. If they took the image of his mom out from the scene at the end they could leave the dialogue in place.
The entire rest of the movie does such an awesome job making every moment do more than one thing in a clear and consistent way that it renders the opener superfluous - and yet the writers couldn't resist.