In any kind of formal writing, the writer should be sure to include a formal tone, or attitude, toward the subject.
While writing the formal letter it is important to address with formal tone since it helps in establishing writer's respect for the concerned person. It doesn't imply the use of informal language because informal language is used with someone who is known personally. Additionally, formal tone helps in asserting the writer's seriousness about an issue or the topic the writer is talking about. All academic writing uses formal writing only.
