What are we without rules? Monkeys that's what. Well, not those super smart monkeys that end up taking over the world and destroying the Statue Of Liberty. We couldn't hope to compete with them.
The finer point of rules is that without properly adhering to them, we end in chaos. If you cross the streams, everything goes to sh*t, if you mess with the timeline, Doc Brown gets really angry (unless it involves him getting laid, then it's fine). There is cause and effect, and as such it's an appealing mechanism to use in telling stories.
Films have long established a framework of rules to guide their stories. Everything from sci-fi to horror comes with a set of laws that make sure their characters can't just do as they please without ramifications. Freddie Kruger can only attack you in your sleep; if the totem spins forever, you're in a dream; Gremlins can't be fed after midnight.
Occasionally those rules are as famous as the characters, which makes it especially annoying when the writers just completely bloody ignore them for the sake of their plot or a few easy thrills. We've already discussed one set of such cases, and it seems there are even more of them out there.
If you wait around long enough, films that rely upon complex rules will end up being hoisted by their own petard.