The following contains minor spoilers for Pacific Rim Uprising.
Movies can go through numerous changes during production, but even after filming is done, some pretty big changes can still happen following the response to test screenings. Small audiences get shown an early version of the movie as a way of gauging whether or not the movie is on the right track. Steven S. DeKnight recently revealed two scenes that were not part of the original cut of Pacific Rim Uprising until tests made it clear they were necessary in order to better flesh out the story the movie was trying to tell. The first sequence that was added was actually the first thing you see in the movie. The opening montage with John Boyega's voiceover was added later because test audiences apparently needed help remembering what happened in the first film. According to DeKnight...
It's understandable that people coming in cold to Pacific Rim Uprising might need a hand. The original Pacific Rim came out back in 2013 and while many might have rewatched the original before going to see the theatrical release, test audiences likely did not. Even fans of the first movie would be most likely to just remember the general action of the first movie without necessarily remembering the details. The prologue also adds information about what has been going on since the last Kaiju attack, giving the audience an understanding of how things have changed since the first movie ended.
However, the prologue wasn't the only thing that was added to Pacific Rim Uprising and the other big change may have been even more important. Steven S. DeKnight tells Collider that they also added a scene to the film in order to more deeply explore the relationship between the characters played by John Boyega and Scott Eastwood.
Scott Eastwood plays the former Jaeger partner of John Boyega's Jake Pentecost who isn't all that happy to see his former teammate when Jake returns to help him train new recruits. However, Eastwood's character isn't a true antagonist and this scene in question helps to show that. Eastwood's character could potentially be unsympathetic and just come across as an ass without it. Instead, it's clear Eastwood's character just wants everybody to do their jobs well, including Jake. It's also one of the funniest scenes in the movie, allowing Boyega to show off some great comedic skill.
Pacific Rim Uprising would likely be a good movie without these scenes, but the addition of them clearly does help things along. While test audiences are often criticised for the way they can drastically impact movies, sometimes, they do help.