What is an applicant's cover letter for?
It is to tell the ECO (Entry Clearance Officer) more about yourself, your relationship with your partner, your intentions and your plans for the future.
Here you are given the chance to convince the ECO that your relationship is genuine, that you intend to marry your fiance and permanently live together.
This is one of the things that is not too technical, there are no rules. Just share your journey with the love of your life.
The first part of my cover letter is here:
http://noreeneunice.blogspot.com/