A cover letter needs to answer a basic question: Why are YOU the BEST person for this SPECIFIC position with this SPECIFIC company? Anything that does not provide such insight does not belong in your cover letter.

Imagine receiving a multitude of resumes for a specific position and trying to discern the seeds from the chaff. The biggest question on an employer's mind as he or she reads each letter is SO WHAT?! By answering these questions effectively — Why do you want to work here? Why should I hire you? What can you do for me, for the company, and for the rest of the staff? — your chances of getting an interview dramatically improves.

A resume can answer these questions, but not without some searching and assumptions on the reader's part. Connect the dots between your experience and skills, and what you think the employer is seeking. If you don't draw the lines, the reader will have to complete the picture for you.