Everything in the original was there because it was a basic story. She was a princess cause it made the mission to find her more interesting for the plot. And young boy fulfilling his destiny to save a princess from capture, and blow up the space station run by the evil empire.
There was nothing more to the story than that, Lucas had some notes and some backround on the characters, but these plans for a huge saga are such malarky, it was originally a standalone movie, that Lucas made Vader survive just in case he could do sequels, in which he had no idea where the story would go from here.
That is why it doesn't make sense if you watch the PT, and then watch the original SW, nothing adds up, cause nothing was there in 1977 yet.