It's a different experience watching a film (or a play, or reading a book, or any kind of story really), after the first time.
You're watching it for different reasons - you don't need to concentrate on plot because you already know it. Generating tension in the audience is an exercise in mood and atmosphere, not suspense and surprise. Needless to say, jump scares are ineffectual. Characterisations are viewed differently, because you can take into consideration things people do that are yet to occur. Even judging things like pacing is difficult, because your audience is already anticipating what's about to happen, and when.
I remember reading some commentary about the so-called director's cut of Blade Runner (the one sans Deckard's voiceover). It was pointed out that everyone that viewed it did so while recollecting the voice-over that they could remember was originally there. So to say that the director's cut was the better version is a very simplistic interpretation of the experience. A similar thing could also be said of the various Alien3 versions.