Good for complicated or long messages. Good for formal communication. It takes some time for your message to be received, by which time the situation and what you want to happen might have changed. A response is delayed until the other person receives the letter, reads it and gets back to you. You aren't able to immediately check if your message has been understood.
Letters can be impersonal.
Influencing tips and hints Be clear about the purpose of your letter. Know what you want to say, about what and to whom. Convey your message in as few words as possible. Make your message as simple as possible - your reader may have many other things competing for their attention. Keep your reader in mind - their skills in the language you are writing in, their needs, their readiness for your message.