For instance, in a typical job application, the problem is that the company or organization needs a position filled, and more importantly they need someone who can do the job well and help the organization achieve its mission (which may be more profits, or it may be to save children in Darfur or to end global warming or to cure cancer). You arouse their interest by telling them that you’re the right person for the position and by promising to help them achieve their mission.
I can’t tell you how many truly awful letters I’ve had to read in my life. As a former administrative assistant, I was responsible for all the unsolicited submissions to a major New York museum. Artists and collectors all over the country (and abroad) wrote to have their work considered for display or acquisition, and to be honest the decision frequently rested more on the quality of their cover letter than on their work _ which, romantic dreams aside, rarely if ever sells itself.