The saying goes that the book is always better than the movie, it's what most people in the book community seem to live by. In most cases, that is true. If you think about it, it also makes sense that the book would be superior. A movie has a two hour time slot to tell a story. Where as a book has hundreds of pages to fill to tell, while also having the means to get inside a characters head and can explore the complexities of their feelings in great depth. A movie has to use visual cues to express how someone might be feeling, or use really bad voice overs (okay sometimes voice overs work, but a lot of times they're really unnecessary or just don't work).
Also, since books have more time to explore a story and characters, it has the time to have a lot more fully developed, three dimensional characters. Since movies have less time, then a lot of the side characters come off as more two dimensional in order to save time. It's not always bad or always the case, but it is pretty common.
There are plenty of cases where a movie adaptation did terrible justice to the book, and fans pray that people who only saw the movie don't just assume the book is complete garbage (Percy Jackson movies, I'm looking at you!). There are cases, like "Harry Potter" and "the Hunger Games," where the adaptation is done really well, and both the book and movie are worthy of of the media they're being told in.
Then, there are times when the movie adaptation is actually better than the book it's based off of (insert audible gasps, cries of despair and "Alyson, are you feeling okay?" comments here).
Even though books are great, not every book is perfect, just like how isn't movie isn't a masterpiece. There have been plenty of books I've read that I didn't like, or I thought were just okay. In cases like those, a movie adaptation has the chance to fix some of the problems the book might've had, or reinterpret something in order to improve it.
The first example that comes to mind of when I think of this topic is the "Maze Runner" franchise. I read the book series back in seventh grade and enjoyed the story, but the book didn't leave that big of an impression on me. And the biggest problem that I had with the series was not liking a lot of the characters. The characters either weren't that likable, or they all sounded the same, or I forgot they existed (sorry Brenda). But when I saw the movies, the characters left a very different impression on me. I remembered all their names (though that probably had to do with the fact that I could actually see all the characters and not try to picture them all in my head), and I could tell you at least one thing about each of them. I also liked every single one of them, even Gally, the guy who was supposed to be the "antagonist."
When the "Harry Potter" movies came out people complained about their favorite scenes getting cut from the movie. While that's annoying, there are times when a subplot needs to be cut and as a result can improve the story. For example, when "Jaws" was adapted to the big screen, the writers decided to cut a subplot from the book involving Mrs. Brody's affair with Matt Hooper. This was a great choice for the movie. The subplot was completely not needed, took up the whole middle of the book and made some of the characters really unlikable. Since it was taken out of the movie, the story felt more focused, because killing the shark is what's important, and all the characters are likable.
So yeah, sometimes the movie can tell a story better than the book can. It's a pretty rare occurrence, but it does happen.