While your resume provides employers a god overview of your skills and experience the cover letter allows you to highlight why you fit the advertised role. The letter is more free form as such allows you to demonstrate your knowledge of the role, the organisation, highlight how your specific skills and experience will add value and finally it allows you to explain any anomalies on the resume.
Another way to think about the cover letter is as the marketing brochure, the resume is a list of specifications, the cover letter highlights …