You could argue dialog is the most crucial element of film and tv sound. You can remove sound fx or music from a movie, and you’ll still have a playable movie (maybe not the most interesting movie – but you can watch it). If you remove the dialog track, it would be a tech problem – not a movie. Poor dialog quality (or mix) can be the difference between the success and failure of a film, too. If you can’t understand what anyone is saying, how long would it be before you watch something else?