Are you referring to the relationship essay where you write about the development of your relationship and your plans for the future?
If yes, it's completely optional. I didn't write anything more than the space provided on the form. My husband wrote nothing. (Thank the heavens, his grammar and spelling are awful.) Some people write letters. Most seem to be 2-3 pages long, although I know of people writing 11 or more pages. One person wrote almost 30, but she included pictures in the text - so if they took a vacation, she had a picture or two from the vacation. (It sounded really cute to be honest.)
So it's up to you. Evaluate your situation. Do you feel you need a letter? Are there special circumstances around your relationship that you need to explain to the VO? (I. e., a Muslim woman marrying a non-Muslim man; significant age difference; you have a doctorate and your spouse is a high school drop out.... )
(I personally had a few "red flags" - everyone has at least one - but I didn't feel they were big enough to warrant explaining.)