Computer-generated imagery can be used in all sorts of situations, from explosive action sequences to smaller, more obscure touch-ups. Whether you notice it or not, almost every single movie you see will contain some sort of digital effects work, and most of the time, these moments slot seamlessly into the movie around them.
In recent years, one of the biggest CGI developments has been the ability to fully recreate deceased actors onscreen, or make older actors look decades younger. As seen in Rogue One, Captain America: Civil War and the upcoming Captain Marvel, this tech is amazing, but the combination of a recognisable actor and some CGI augmentations doesn't always work out for the best.
Changing (or creating) humans with visual effects is one of the hardest tricks to pull off, because if there's even a slight problem with the way the character looks or moves, it can ruin the audience's investment in a scene, or even the entire movie.
Sometimes though, effects like these are done so badly that we can at least have a good ol' laugh at them, despite how much worse they make their films.