I love movies. Ever since I was a student at secondary school, I´ve been watching lots of great productions and all-time classics (with some TV series thrown in).
When I started watching movies, I could only get a vague idea of the plot, even if I turned on the subtitles. As my English improved over the years, I no longer watched my favourite movies with subtitles (who wants to "read" a movie?). These days, unless the accent is too strong (some movies are difficult to follow even for native speakers), I can understand and enjoy them from beginning to end.
I will now tell you why advanced students should watch movies in original version:
Movies are made for native speakers, not learners. This means they include grammar, vocabulary and dialogue relevant to the plot and themes of the movie rather than language adapted to English learners. The actors, therefore, speak as they would in real life. As an advanced student (C1), you should always watch movies without subtitles.
When you learn English at school, teachers usually speak to you slowly and clearly so that you can understand what they say. In everyday English conversations, however, the speech is not like that at all. Similarly, actors say their lines in as natural a way as possible, so their speech is usually fast. If you don't get used to fast speech, you will never be able to effectively follow and take part in a conversation with native speakers. That is why watching movies is excellent practice to become familiar with that kind of speech.
Watching movies in its original version will expose you to a variety of English accents; you can even hear several accents in the same movie. The more accents you are exposed to, the easier it will be for you to understand and communicate with speakers from around the world. If you only listen to, say, British English, you may have some difficulty understanding people from America, South Africa, Australia, Scotland, etc. The beauty of movies is that they allow you to listen to people from all those countries (or at least to people pretending to be from those countries!).
Slang and colloquialisms
Some movies include lots of slang terms and colloquialisms. This is a kind of language that you are unlikely to learn at school, and yet, it is very common in everyday informal conversations. Some of the slang in movies may even not be listed in dictionaries or only in specialized works.
When you watch movies, you're not only having a good time and improving your English, you're also learning about culture. Movies in English reflect the customs, traditions and way of life of the people living in certain countries or regions. For instance, you may have seen in some movies (American Pie, There's Something about Mary, etc.) that high school students in America have a formal dance at the end of the school year called "the prom". Or how every 4th of July people celebrate the Declaration of Independence. So, by watching movies you will also learn about the way Americans, Britons, Australians and other English-speaking people live and organize their lives.
Watch, enjoy and learn
When you do something enjoyable you are more likely to remember what you are doing. Likewise, if you watch movies you really like, you will be able to assimilate and remember more of its vocabulary and grammar than by doing, for example, some boring exercises. This, in turn, will increase your motivation to learn more so you will be continually improving your English.