Among other thing, letter bring news, establish relationships, ask for things, and make suggestions. Letters can be teacher-to-student(s), student-to-student, student-to-teacher, pen-friend letters, or letter created for a specific situation or purpose.
Letters can come from a friend, from a boyfriend or girlfriend, from a person in authority (e.g. the Mayor of the town), from an intergalactic empire, from a character in a tale, etc. The teacher has to judge what kind of letter will appeal most to the student.