Totally, films usually have a time limit and so they can't really add everything they wanted to. And depending on the director/writer it's better to read the book in case they don't explain things or they do in the film but you understand a lot by reading the book.
I'm really into the dark stories that they were originally. Or like things they just don't add as you said. There's an anime I've watched and I really like it and went back to read the manga, and found some really funny and interesting chapters that didn't get an episode. Ah really~?! I hadn't known, that's really interesting!