Yes, I wrote one to a therapist/pdoc/counselor I saw during my first year of college. (It was designed to be written but not sent.) T thought I was having trouble feeling my sadness over his loss with him right there in session, because I had so many good feelings towards him (my current T). Kind of like transference preventing me from mourning a past loss. So he had me write a letter away from his presence, so I could better let the sadness in. This really helped a lot.