A good cover letter motivates the potential employer to read the attached resume and subsequently invite you to interview for the position you are seeking.
Many employers receive hundreds of cover letters and resumes each day.
Most employers can interview only a small fraction of the people who write requesting interviews Employers often use cover letters as the initial screening tool in deciding whom to interview. As with a resume, a cover letter is not designed with the goal of obtaining a job offer initially. All you want the cover letter to do is to help you get your foot in the door. To an extent, the cover letter expands upon the resume, but it does one additional thing. It adds personal flavor to your approach.