In a cover letter you sort of answer a hidden question that lies in the readers' minds regarding why they should be reading you and even more why they should keep reading further ..
In order to make one do what you wanna them do for you, you need to make them know why should they be interested in you ..
So in order to make them seriously consider what you are proposing for or offering, you really should make the reader feel in need for what you have to offer and this can only be made if you can put what you already have to offer in a good shape and form to suit what the needs of your audience are ..
Best Luck!
Wise
