Film and cinema industries, for example, issue these kinds of warnings to ensure they comply with the law. They know they'll face penalties otherwise, and children may not care much about them.
A 12+ warning for profanity is unnecessary. In fact, it might be more beneficial for children to learn about profanity in movies. After all, they'll encounter these profanities more often as they get older.

I watched a horror movie at home before, when I was a kid. I wasn't old enough to watch that movie, and I should have heeded the warning.