Letter Paragraphs: Professional letter paragraphs should be relatively concise. The first paragraph may include a brief friendly opening and a concise explanation of your reason for writing. The second paragraph (and any subsequent paragraphs) should expand upon your reason for writing. The final paragraph should restate your reason for writing and, if applicable, state your plan of action (or request some type of action be taken by the reader).
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.