Letters are only required to explain anomalies in one's record, n'est pas?
Letters to the board are used either to explain anomalies in the FITREP record, or to get information to the board that for one reason or another didn't make it to the FITREP. All correspondence to the board must be briefed in full, and if a record is a little unusual, a letter can serve as a "speed bump", essentially forcing the members to spend more time on that record. I recommend a minimalist approach...a one page letter is enough to explain some unusual circumstances in a FITREP. Submit a letter if you have picked up a particular AQD or an academic degree that doesn't show on your last FITREP. Be thoughtful about what you submit. Submitting certificates for on-line courses that don't affect your primary job isn't useful, and is probably detrimental. Your briefer will have to review and brief every piece of paper you submit to the board every time your record comes through the tank.
Letters can be a double-edged sword. Don't put anything in a letter that you don't want out there, particularly medical information. I'd also suggest floating any letter past your detailer for his take on it before submitting it to the board.
R/