Shelley L. answered 07/15/20
Successful filmmaker, screenwriting instructor and acting coach
Dialect coaches are invaluable to filmmakers. What happens when an Englishman watches a film where an Australian actor doesn't have a credible British accent, one that is passable for the region and socioeconomic class he is portraying, is that the viewer can't suspend disbelief and is distracted. That is, he is taken out of the movie experience and instead he's thinking about that awful accent. The director wants his audience to enjoy a film's full ride with no stops,. And the actor needs to give a credible performance to expand the kinds of roles he can play, and the quality of the films he will be offered.
Besides learning proper diction for a role, actors need to learn not only the correct dialect, but also know the body language and the way people speak in that corner of the world; a really good dialect coach will teach this as well.
Now when an actor knows he needs the help of a dialect coach but there is no one to help him, he may hire one on his own. This is not unusual. A responsible actor will always look for additional guidance when the producer or director doesn't provide it.