If everyone wrote a script and shared it with someone to get honest feedback, we all would realize 1) how incredibly difficult it is to get the essence of what's in your head down on the page, and 2) how hard it can be to make sure the reader is actually getting what you want them to. When you take a script through development into production, you realize the bizarre fact that many talented and smart people can read exactly the same thing and come away with a different understanding about what it is, and you also realize that there's really only one person who understands the story from absolute beginning to end (and the journeys the various characters undergo): the writer. If you're very lucky you might have a producer and/or director who's worked through the story with you, but that's not a guarantee at all. And most of all, you realize that from a writer's perspective, the most "perfect" form of the story is that final refined draft that you struggled through and slaved over, before you started to deal with notes from producers, execs, actors, directors, etc. Usually it's all downhill from there (the occasional miracle aside), so you'd better make sure it's as damn perfect as it can possibly be, because the very process of grinding through production will force concessions or compromises. We all make the assumption that every facet of a movie is intended to be the exact way it appears on screen... but in fact it could be many things: a fortunate accident, an unfortunate accident, a re-arranging of a shot or scene that was never originally intended, a glimpse of a reaction from before "action" or after "cut". Sometimes it's just an unavoidable bump that the filmmakers try to minimize -- after realizing in post that a moment isn't playing the way they thought it would, and there's no alternative and no money left to try and shoot a replacement.
So in an odd way, that script that was sold or greenlit is both vital to the final movie yet probably not that similar to it as well.