Reshoots are often treated with suspicion by people not in the movie business. They take it as a sign a movie is in serious trouble and desperately needs fixing. But the truth is most major movies build reshoots into their schedule. The studio reviews the footage shot in principal production and find they're missing certain pieces that would make it flow better. This could be as small as adding insert or reaction shots or filming a whole new action scene or subplot. But if the reshoots are pulled off correctly, an audience wont be able to tell the difference.
That said, there are plenty of well documented movies that really were in trouble and undertook severe reshoots to try and fix the problems. In some cases the movies are already unfixable but the studios have to try and salvage their investment somehow. So they'll throw money at it to patch up problems that could have been solved the first time if they'd just taken time to develop it properly. They'll hire new writers, editors and directors to seal up the cracks.
Sadly, it rarely helps the final product. Since they didn't, what's usually left is an uneasy mix of tones and plots that never quite gel together or connect with an audience.
Here are some of the most obvious examples where the stitches are still showing.