Simply put, no other medium has the reach that movies do. So when a major Hollywood blockbuster like "La La Land" comes along with Ryan Gosling playing a struggling jazz pianist, that can only be a good thing for helping to spread the word about jazz in general.
Who knows how many young people are going to hear the word jazz for the first time as a result of seeing the movie? Or be curious to enough to learn more about it, maybe even enroll in a jazz combo at school or take piano lessons? Hopefully films like "La La Land" will do just that.