Farewell Letter is a letter used commonly in the business field. It's usually to say goodbye because of leaving the current work. It can be formal or informal. The purpose of the farewell letter is to make an official farewell and show respects to the coworkers or boss. It's optional to write a farewell letter, but it's better to write one, especially leaving a company you have worked for a long time. You can read some samples to know how to write a farewell letter.