The goal of the cover letter is to bridge the facts and experiences on your resume with the position for which you are applying.  In other words, it should connect the dots for the employer so they don’t have to think too hard about how you might be qualified.  Many find success organizing their cover letter around a few key skills they want to highlight and then backing them up with proof from experience.  Because the cover letter is also an example of your writing abilities, you should spend quality time on it.