A letter is appropriate when you want someone to fully digest your thoughts. Excessive talking had the potential to choke them. They will vomit your words and that energy will be wasted. The written word allows for the meal to be regurgitated and re chewed numerous times until all the meaning is extracted. Small conversations should proceed from these key points.
When dialogues become separate monologues, it may be time to try another form of communication. Some wrongly believe that written language is cowardly. This belief has some merit, but if the goal is to effectively communicate- why not start at a point that is comfortable for both parties. You can erase a sentence, but you can’t erase words already spoken. Still, in the end a face to face is necessary, but hopefully then the excessive negative emotion can be replaced with rational minded individuals who are skilled enough to participate in the art of communication.