Think of your cover letter and resume as sales tools.
All too often I see a real lack of understanding of the effective use of these important documents. Understanding the purpose, use and integration with each other is vital for achieving positive results. They are not, nor should be, a regurgitation of your jobs, titles, experiences, and other past information looking backwards with few if any positive results.
In my opinion, it is critical that you understand that these documents are your Sales Brochures and need to be targeted/focused with your features and benefits. Highlighting your successes, achievements and results is important so you potentially can stand out from the other candidates and receive positive consideration.
Integrating your cover letter and resume is very important. Your cover letter plays the role of introducing yourself and resume, while your resume becomes your targeted Sales Brochure. Your cover letter is really building the bridge “I have what you need” “I am what you want” type of statements that are important in moving the reader to your resume aka Sales Brochure.
Mirroring back the key issues contained in the job description in both your cover letter and Sales Brochure dramatically increases the potential of making it through the Applicant Tracking System and makes it so much easier for the human reader to see and feel you have what they are looking for in the 10-15 seconds generally given to resume reviews.
Bottom line, it is your responsibility to tell the world how wonderful you are and using the matrix of what, how and results allows you to do so with credibility.