While smooth pre-production and shooting phases are both highly desirable, most films are truly found in the edit bay. You can have some great performances and a multitude of eye-catching camera shots, but until all that footage is pieced together, you don't really know how well the film is going to play.
Some films are even edited while they're playing in cinemas, although this isn't always for artistic reasons. Sometimes, yes, a filmmaker will simply want to make their movie better after seeing how audiences have responded to it, but post-release edits can also take place as a result of controversy, censorship, or even health concerns.
It's rare for these changes to impact the film in a major way, with most of these edits being minor nips and tucks to rectify a small issue, or have small chunks of footage cut from the film.
Still, these changes exist, and chances are, you've seen the original, unmodified versions of some of these movies, due to the fact that they screened to the public for a limited time before being altered to the version we can all watch today.