Yes, there are. Some movies draw inspiration from various sources but not directly from comic books. For example, 'The Truman Show' has elements of a fantastical world but isn't based on a comic.
The comic might expand on the story or offer additional perspectives. It could have scenes that weren't in the movie or give more depth to the characters.
Definitely. The film Constantine takes inspiration from the comic and incorporates many elements and storylines from it. However, there are some creative changes made to adapt it for the big screen.