Watching the movie only is the lazy option. Reading the book allows you to see the characters and the scenes in your own way, in your own light. You get to direct your own movie, as Pemworth said. You also get to see how the true storyline goes. Most directors change scenes and sometimes delete them. They also add their own scenes. The book shows how the story was meant to go, how the author wanted it to be. You can miss a lot if you just watch the movies.
Watching the movies is fine, but it is better to read the books beforehand. Reading is also better for you.