My schools health advising office has this on their site.
What is a Letter of Intent and should I send them?
A Letter of Intent is correspondence to professional schools letting them know that, if they were to accept you, it would be your intention to attend their school - a promise note, of sorts. It has been made popular by the student doctor network. The HPAO does not specifically recommend such a letter, as many admissions Deans have made it clear that they are not all that useful. What is useful, however, are well timed updates to admissions offices, indicating your continued interest in the school and any new activities, relevant experiences you have had since your original application. Certainly, at a minimum, you should send updated transcripts (above), but this is also a good time to send a note letting the admissions office know that the transcript is on the way, you are still interested, and you have had these new experiences.
If you are placed on a waitlist, it is also a good idea to acknowledge this update to your status in writing and to indicate that you are pleased to still be under consideration and that you remain very interested.